Lateral Magma Flow in Mafic Sill Complexes Craig Magee1, James D

Home , Cone sheet, Igneous intrusion, Mackenzie dike swarm, Sill (geology)

Research Paper

GEOSPHERE Lateral magma flow in mafic sill complexes Craig Magee1, James D. Muirhead2, Alex Karvelas3, Simon P. Holford4, Christopher A.L. Jackson1, Ian D. Bastow1, Nick Schofield5, Carl T.E. Stevenson6, Charlotte McLean7, William McCarthy8, and Olga Shtukert3 GEOSPHERE; v. 12, no. 3 1Basins Research Group, Department of Earth Science and Engineering, Imperial College London, London SW7 2BP, UK 2Department of Geological Sciences, University of Idaho, 875 Perimeter Drive, Moscow, Idaho 83844, USA doi:10.1130/GES01256.1 3Schlumberger Multiclient, Schlumberger House, Buckingham Gate, West Sussex RH 6 0NZ, UK 4Australian School of Petroleum, University of Adelaide, North Terrace, Adelaide SA 5005, Australia 5Department of Geology and Petroleum Geology, School of Geosciences, University of Aberdeen, Meston Building, Old Aberdeen, Aberdeen AB24 3UE, UK 17 figures; 1 table 6School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 7School of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Glasgow G12 8QQ, UK CORRESPONDENCE: c.magee@imperial.ac.uk 8Department of Earth Sciences, University of St Andrews, Irvine Building, St Andrews KY16 9AL, UK

CITATION: Magee, C., Muirhead, J.D., Karvelas, A., Holford, S.P., Jackson, C.A.L., Bastow, I.D., Scho field, N., Stevenson, C.T.E., McLean, C.,McCarthy, ABSTRACT 2005; White et al., 2008; Ebinger et al., 2010); (2) total melt volume estimates, W., and Shtukert, O., 2016, Lateral magma flow which can be used to assess the thermomechanical state of the mantle (White in mafic sill complexes: Geosphere, v. 12, no. 3, The structure of upper crustal magma plumbing systems controls the dis- et al., 2008; Ferguson et al., 2010); (3) the physiochemical evolution of magma p. 809–841, doi:10.1130/GES01256.1. tribution of volcanism and influences tectonic processes. However, delineating (e.g., Holness and Humphreys, 2003; Cashman and Sparks, 2013); (4) host-rock the structure and volume of plumbing systems is difficult because (1) active in- deformation induced by magma intrusion, which may be expressed at the Received 26 August 2015 Revision received 29 January 2016 trusion networks cannot be directly accessed; (2) field outcrops are commonly Earth’s surface (e.g., Wright et al., 2006; Biggs et al., 2009; Pagli et al., 2012; Accepted 10 March 2016 limited; and (3) geophysical data imaging the subsurface are restricted in areal Jackson et al., 2013; Magee et al., 2013a; Tibaldi, 2015); and (5) volcanic erup- Published online 28 April 2016 extent and resolution. This has led to models involving the vertical transfer tion locations and styles (e.g., Abebe et al., 2007; Gaffney et al., 2007; Mazza of magma via dikes, extending from a melt source to overlying reservoirs rini, 2007; Tibaldi, 2015; Muirhead et al. 2016). However, the inaccessibility of and eruption sites, being favored in the volcanic literature. However, while active plumbing systems and limitations in exposure at the Earth’s surface there is a wealth of evidence to support the occurrence of dike-dominated makes reconstructing the geometry and connectivity of regionally extensive systems, we synthesize field- and seismic reflection–based observations and intrusion networks from field outcrops challenging. highlight that extensive lateral magma transport (as much as 4100 km) may In order to delimit plumbing systems, it is therefore important to inte- occur within mafic sill complexes. Most of these mafic sill complexes occur grate field observations with data and imaging techniques that probe magma in sedimentary basins (e.g., the Karoo Basin, South Africa), although some in- pathways and storage in the subsurface (Tibaldi, 2015). Indirect analytical ap- trude crystalline continental crust (e.g., the Yilgarn craton, Australia), and con- proaches commonly involve: (1) the examination of ground deformation pat- sist of interconnected sills and inclined sheets. Sill complex emplacement is terns (e.g., using InSAR, interferometric synthetic aperture radar) inferred to largely controlled by host-rock lithology and structure and the state of stress. relate to the emplacement of magma (e.g., Pedersen, 2004; Wright et al., 2006; We argue that plumbing systems need not be dominated by dikes and that Pagli et al., 2012; Sparks et al., 2012); (2) scrutiny of petrological and geochem- magma can be transported within widespread sill complexes, promoting the ical data to assess magma contamination, residence times, crystallization his- development of volcanoes that do not overlie the melt source. However, the tories, and melt source conditions (Cashman and Sparks, 2013, and references OLD G extent to which active volcanic systems and rifted margins are underlain by therein); and/or (3) mapping the location and crude geometry of crystallized sill complexes remains poorly constrained, despite important implications for intrusions or present-day zones of melt using geophysical techniques such elucidating magmatic processes, melt volumes, and melt sources. as potential field, magnetotellurics, and seismicity (e.g., Cornwell et al., 2006; Whaler and Hautot, 2006; Desissa et al., 2013). For example, the application OPEN ACCESS and synthesis of these techniques in the East African Rift system and Iceland INTRODUCTION have greatly improved our understanding of magma-tectonic interactions and allowed processes controlling continental break-up, such as dike intrusion, to Interactions between tectonic and magmatic processes, as well as the be analyzed in real time (e.g., Ebinger et al., 2010; Gudmundsson et al., 2014; lithology and preexisting structure of the host rock, control the geometry and Sigmundsson et al., 2015). It is, however, important to consider that these data connectivity of magma conduits and reservoirs (i.e., a magma plumbing sys- poorly constrain intrusion geometries and are commonly interpreted within This paper is published under the terms of the tem). Delineating plumbing system structure can therefore provide important the classical framework of igneous geology; i.e., magma migration within the CC‑BY license. insights into: (1) continental rifting (e.g., Ebinger and Casey, 2001; Kendall et al., Earth’s crust is generally facilitated by the vertical intrusion of dikes, extending

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from a deep magma reservoir or melt source to overlying shallow-level intru- Mountain laccoliths, Utah, USA, Johnson and Pollard, 1973; Torres del Paine sions and volcanoes (Fig. 1A; Tibaldi, 2015, and references therein). laccolith, Chile, Michel et al., 2008), which commonly form single or com While numerous studies document vertical magma transport in dike-domi posite, thick (typically >1 km) laccoliths and plutons with limited lateral extents nated volcanic plumbing systems (e.g., Kendall et al., 2005; Keir et al., 2006; ( 20 km), differs significantly from that of mafic sill complexes (e.g., Cruden Wright et al., 2006; Corti, 2009; Bastow et al., 2010; Cashman and Sparks, and McCaffrey, 2001; Corazzato and Groppelli, 2004; Menand, 2008; Bunger ≲ 2013; Muirhead et al., 2015; Tibaldi, 2015), it is also widely accepted that lateral and Cruden, 2011). Mapping of flow indicators within sill complexes reveals magma flow can occur either within individual plumbing systems where vol- that interconnected networks of mafic sills and inclined sheets can facilitate canoes may be laterally offset by as much as ~20 km from their magma reser- magma transport over distances of as much as 12 km vertically and 4100 km voir (e.g., Fig. 1A; e.g., Curtis, 1968; Cartwright and Hansen, 2006; Gudmunds- laterally (Cartwright and Hansen, 2006; Leat, 2008) and could thus feed vol son, 2006; Aoki et al., 2013; Aizawa et al., 2014; Muirhead et al., 2014; Tibaldi, canic eruptions that are laterally offset considerable distances from the source 2015) or giant radiating dike swarms, which can facilitate lateral magma flow of mantle or lower crustal melting (Fig. 1B; Magee et al., 2013b; Muirhead over hundreds to thousands of kilometers (e.g., Ernst and Baragar, 1992; Ernst et al., 2014). The consideration that a plumbing system may be characterized et al., 1995; Macdonald et al., 2010). Recent field-, seismic reflection–, and by a laterally extensive sill complex, as opposed to a network of vertical dikes, modeling-based research further indicate that plumbing systems at shallow could therefore have broad implications for the interpretation of geochemi- levels (typically <3 km depth), particularly those hosted in sedimentary basins, cal, petrological, and geophysical data obtained from volcanic settings (e.g., may instead be characterized by extensive sill complexes (e.g., Chevallier and Meade et al., 2009). For example, recent field- and seismic-based studies have Woodford, 1999; Smallwood and Maresh, 2002; Thomson and Hutton, 2004; suggested that surface deformation, generated by sill and sill complex em- Planke et al., 2005; Cartwright and Hansen, 2006; Kavanagh et al., 2006, 2015; placement, may not directly correlate to the location and volume of intruding Lee et al., 2006; Leuthold et al., 2012; Leat, 2008; Menand, 2008; Polteau et al., magma if plastic deformation of the host rock occurs (Jackson et al., 2013; 2008a; Cukur et al., 2010; Bunger and Cruden, 2011; Gudmundsson and Løtveit, Magee et al., 2013a, 2013b; Schofield et al., 2014). 2012; Muirhead et al., 2012; Svensen et al., 2012, 2015; Jackson et al., 2013; To emphasize the potential importance of lateral magma flow within sill Magee et al., 2014; Zhao et al., 2014; Button and Cawthorn, 2015; Schofield complexes, the aim of this contribution is to synthesize new and published et al., 2015). Sill complexes imaged in seismic data and observed in the field data from mafic sill complexes around the world (Fig. 2). We examine differ- extend laterally for tens to thousands of kilometers and are dominated by an ent tectonic settings, temporal variations in sill complex emplacement rela- interconnected network of mafic, relatively thin (typically <100 m thick) sills, tive to tectonic activity (e.g., synrift and postrift intrusion), durations of sill which frequently exhibit saucer-shaped morphologies, and inclined sheets complex emplacement, and various melt sources (e.g., mantle plumes and/or with only a small proportion of dikes (e.g., Fig. 1B; e.g., Cartwright and Hansen, small-scale mantle convection; Fig. 2). We highlight that (1) sill complexes can 2006; Leat, 2008; Muirhead et al., 2014). In contrast, the structure of shallowly facilitate lateral magma flow (e.g., Leat, 2008), with little or no accompany- emplaced (<3 km), intermediate to felsic plumbing systems (e.g., the Henry ing surface deformation (Jackson et al., 2013; Magee et al., 2013a; Schofield

AB Map-view Figure 1. Schematic representation of magma plumbing systems. (A) The traditional perspective, whereby volcanoes are fed via dikes and directly over the Upper melt source and shallow-level reservoirs. e crust (B) Upon entering a sedimentary basin, a laterally extensive sill complex forms that Conrad may feed volcanoes that are laterally off- discontinuity set from the melt source. Lower crust Crustal profil Sedimentary Moho basin

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Rockall Basin Faroe-Shetland Basin Møre & Vøring basins Setting - Rift basin Setting - Rift basin Setting - Rift basin Intrusion age - Paleogene Intrusion age - Paleogene Intrusion age - Paleogene Host rock - Shale Host rock - Shale Host rock - Shale Melt source - Plume Melt source - Plume Melt source - Plume

? C Raton Basin Setting - Foreland basin Intrusion age - Late Cretaceous Host rock - Coal S I Melt source - Rifting?

NW Australia Shelf Setting - Rift basin Intrusion age - Late Jurassic A Host rock - Shale Melt source - Plume? Rifting?

B Yilgarn Craton Setting - Craton NZ Karoo-Ferrar Large Igneous Province Intrusion age - Proterozoic Setting - Rift basins Host rock - Crystalline rocks Intrusion age - Jurassic (183 Ma) Melt source - Plume Host rock - Sst, Shale, Coal Passive margin Melt source - Plume Ceduna Sub-basin Setting - Rift basin Volcanic rifted margin Intrusion age - Eocene (42 Ma) Host rock - Sst, Shale, Coal Non-volcanic rifted margin Melt source - Intra-continental hotspot?

Figure 2. Map showing the global distribution of continental margin styles, the locations of the case studies reviewed here (gray circles), other offshore areas containing sill complexes (white circles), and onshore examples of sill complexes (stars) (Aspler et al., 2002; Svensen et al., 2004, 2012; Leat, 2008; Polteau et al., 2008a; Corti, 2009; Bedard et al., 2012; Schofield et al., 2012a, 2015; Duraiswami and Shaikh, 2013; Jackson et al., 2013; Magee et al., 2013a, 2014; Wyche et al., 2013; Agirrezabala, 2015; Button and Cawthorn, 2015; Evenchick et al., 2015). Highlighted offshore areas: EG—east Greenland; WG—west Greenland; B—Brazil; S—Senegal; A—Angola; I—India; C—China; NZ—New Zealand. Sst—sandstone.

et al., 2014); (2) the majority of sill complexes occur in sedimentary basins and Tibaldi, 2015); (3) faults can limit the lateral extent of sill complexes and pro- their emplacement is primarily related to the intrusion of magma along me- vide important magma flow pathways (Delaney et al., 1986; Bedard et al., 2012; chanical discontinuities such as bedding planes and/or compliant lithologies Magee et al., 2013c); (4) magma rheology and chemistry appear to influence (e.g., Mudge, 1968; Gretener, 1969; Einsele et al., 1980; Gudmundsson, 1986, intrusion network structure (Cruden and McCaffrey, 2001; Bunger and Cruden, 1990, 2011; Annen et al., 2006; Kavanagh et al., 2006; Menand, 2008; Thomson 2011); and (5) eruption sites may be laterally offset from the melt source and and Schofield, 2008; Schofield et al., 2010, 2012a; Magee et al., 2013a; Barnett shallow-level magma reservoirs (Magee et al., 2013b). In contrast to dike-fed and Gudmundsson, 2014; Button and Cawthorn, 2015; Kavanagh et al., 2015; volcanoes, the distributions of which are commonly used to map zones of

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lower crustal and/or mantle melting or magma storage (e.g., Cashman and intrusions describe a range of morphologies that can be categorized, broadly, Sparks, 2013; Annen et al., 2015), the dispersal of eruption sites associated into strata concordant, saucer shaped (i.e., a flat inner sill that passes laterally with a sill complex may not be a reliable indicator of magma generation zones. into an encompassing, or partially encompassing, inclined limb; Thomson and Hutton, 2004; Magee et al., 2014), and inclined sheet geometries (Fig. 3; e.g., Thomson and Hutton, 2004; Planke et al., 2005; Magee et al., 2014); (2) dif- APPLICATION OF SEISMIC REFLECTION DATA TO ferent host-rock deformation mechanisms accommodating intrusion can be UNDERSTANDING PLUMBING SYSTEMS interpreted (e.g., Jackson et al., 2013; Magee et al., 2013a); and (3) magma flow pathways can be mapped across entire intrusion networks (e.g., Thomson and High-quality marine seismic reflection data have revolutionized our under- Hutton, 2004; Schofield et al., 2012b, 2015; Magee et al., 2014). Despite these standing of sill complex emplacement because they allows subsurface intru- advances, seismic interpretation is an underutilized tool in igneous-based re- sion networks to be imaged in three-dimensions (3D) at resolutions of tens of search and remains an unfamiliar technique to many Earth scientists in the meters (e.g., Smallwood and Maresh, 2002; Thomson and Hutton, 2004; Cart- volcanic and magmatic community. It is therefore worth briefly considering wright and Hansen, 2006; Magee et al., 2014). Studies applying seismic tech- the seismic interpretation methodology of igneous rocks, prior to the analysis niques to analyzing plumbing systems have shown that (1) seismically imaged of plumbing systems imaged in seismic data.

Seismic reflection data Field examples 3D/Map view Cross section A –2.2 SW NE (s) –2.4 Section TW T

Sill N Sill

VE ≈ 5 Strat-con. sill VE ≈ 5 ~1.5 km 0.5 s (TWT) 2 km ~0.1 km TWT (s) W E B - + Figure 3. (A) Strata-concordant (Strat-con.) Section sills observed offshore southern Australia (see Jackson et al., 2013) and in the Theron –2 Mountains, Antarctica (field photo courtesy of Donny Hutton). 3D—three dimen- –3 Sill sional. Vertical axes in seismic images are in two-way traveltime (TWT) and are verti-

TW T (s) Sill cally exaggerated (VE). (B) Saucer-shaped N N sills observed in seismic data from the Exmouth subbasin, northwest Australia VE ≈ 2 Saucer-shaped sill VE ≈ 5 (modified from Magee et al., 2013a) and

~0.5 km 0.2 s (TWT) -2.73 1 km ~2 km NW SE the Golden Valley Sill in South Africa C Inclined sheet (Google Earth image). (C) Inclined sheets in the Rockall Basin (modified from Magee –4.8 et al., 2014) and Antarctica. (D) Laccoliths observed offshore southern Australia

Sill 0.2 s (TWT –5.0 (modified from Jackson et al., 2013) and in the Henry Mountains, Utah (USA).

TW T (s ) –5.2 N VE ≈ 2 Inclined sheet Section ~1 km VE ≈ 5 1 km ) ~100 m D TWT N NW SE (s)

–2.3 ) –2.35 ~1 km Laccolith Eroded upper contact Section Laccolith 0.2 s (TWT VE ≈ 10 Laccolith ~1 km VE ≈ 10 2 km Host rock

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Sill complexes are relatively well imaged in seismic data because igneous Constraining Magma Flow Patterns in Outcrop and Seismic Data rocks commonly have greater densities and seismic velocities compared to the surrounding sedimentary strata. These physical differences at the intrusion– Because this review assesses both field- and seismic-based observations host rock contacts result in a high acoustic impedance contrast (Smallwood pertaining to the role of lateral magma flow in sedimentary basins, it is im- and Maresh, 2002), which reflects more seismic energy back to the surface portant to describe how magma flow patterns can be mapped, particularly than low impedance boundaries typically occurring between sedimentary in seismic data. Within field-based studies there is a long history of using rocks (Brown, 2004). In seismic data, igneous bodies are therefore commonly mineral and vesicle alignment and imbrication, measured using petrological expressed as high-amplitude reflections (e.g., Symonds et al., 1998; Small- or magnetic techniques, to delineate magma flow in sheet intrusions (e.g., wood and Maresh, 2002; Planke et al., 2005; Magee et al., 2015). The geometry Knight and Walker, 1988; Tarling and Hrouda, 1993; Launeau and Cruden, 1998; and seismic-stratigraphic relationships of high-amplitude reflections can be Tauxe et al., 1998; Gudmundsson and Marinoni, 1999; Archanjo and Launeau, used to constrain whether they have an igneous origin or not (Symonds et al., 2004; Callot and Geoffroy, 2004; Cañón-Tapia, 2004; Cañón-Tapia and Chavez- 1998; Smallwood and Maresh, 2002; Planke et al., 2005; Magee et al., 2015). In Alvarez, 2004; Féménias et al., 2004; Liss et al., 2004; Philpotts and Philpotts, particular, high-amplitude reflections attributed to igneous intrusions are com- 2007; Cañón-Tapia and Herrero-Bervera, 2009; Magee et al., 2012a; Neres monly restricted in their lateral continuity and/or crosscut the host-rock strata et al., 2014). Such mineral alignment analyses have shown that the long axes (e.g., Mihut and Müller, 1998; Magee et al., 2013c, 2014). There are, however, of intrusive steps, bridge structures, and magma fingers, which are commonly two major problems with determining whether high-amplitude reflections superimposed onto the larger scale sheet intrusion morphology, correlate to correspond to and accurately define igneous intrusions: (1) it is often difficult the primary magma flow axis (Fig. 4; e.g., Airoldi et al., 2012; Magee et al., to test the validity of seismic-based interpretations of subsurface structures 2012a; Schofield et al., 2012a; Hoyer and Watkeys, 2015). These flow indicators because they are rarely intersected by boreholes; and (2) most intrusions are form because sheet intrusions typically do not initially intrude as continuous expressed in seismic data as tuned reflection packages (e.g., Figs. 3A, 3C), bodies of magma (Fig. 4; e.g., Rickwood, 1990; Hutton, 2009; Schofield et al., whereby reflections from the upper and lower intrusion contacts cannot be 2012a). Instead, the formation of a sheet intrusion is commonly preceded by distinguished. Tuning occurs when the vertical intrusion thickness is between the propagation of thin, laterally restricted magma segments, which may be the limit of separability and the limit of visibility of the seismic data, causing vertically and/or laterally offset from each other (Fig. 4; e.g., Rickwood, 1990; reflections emanating from the upper and lower intrusion contact to interfere Hutton, 2009; Schofield et al., 2012a). With sustained magma input, these seg- (Widess, 1973; Smallwood and Maresh, 2002; Brown, 2004; Hansen et al., ments inflate and coalesce to form a continuous sheet intrusion (Fig. 4; e.g., 2008; Magee et al., 2015). This tuning response promotes uncertainty in the Rickwood, 1990; Hutton, 2009; Schofield et al., 2012a). interpretation of the true intrusion geometry because it is difficult to determine The magma flow indicators (e.g., intrusive steps) described here occur the difference between real features and geophysical artifacts (Smallwood and at scales ranging from centimeters to tens of meters and can be observed in Maresh, 2002; Magee et al., 2015). Several studies have, however, used bore- outcrop and seismic data (Fig. 4; Schofield et al., 2012a, 2012b; Magee et al., hole data to corroborate that interpreted magmatic and volcanic bodies are 2013c, 2014; Rui et al., 2013). Synthetic seismic forward modeling suggests igneous (e.g., Smallwood and Maresh, 2002; Peron-Pinvidic et al., 2010; Grove, that intrusive steps, bridge structures, and magma fingers may manifest in 2013; Rateau et al., 2013; Schofield et al., 2015). Synthetic seismic forward seismic data as minor vertical offsets and corresponding amplitude varia- models, designed to test the seismic expression of igneous intrusion proper- tions in tuned reflection packages (Magee et al., 2015). Seismic- and field- ties observed in the field, can also be utilized to assess the validity of seismic based studies have also suggested that individual sills commonly consist interpretations (Magee et al., 2015). of multiple magma lobes (e.g., Fig. 5A) that represent the incremental em- One limitation of seismic data is that subvertical dikes are rarely imaged placement of discrete magma injections and may contain and/or be bound (e.g., Thomson, 2007). The lack of dike imaging, which is attributed to the low by intrusive step, bridge structures, or magma fingers (Thomson and Hutton, amount of acoustic energy that reflects back to the surface off the subvertical 2004; Hansen and Cartwright, 2006a; Schofield et al., 2010, 2012b; Magee wall of a dike, therefore biases seismic-based interpretations of plumbing sys- et al., 2012b). Mapping the long axes of flow indicators, including lobe-lobe tems. The role of dikes within sill complexes can, however, be investigated contacts, therefore provides insights into magma flow patterns within indi- from (1) field analyses (e.g., Muirhead et al., 2014); (2) the interpretation of vidual intrusions and allows magma input zones to be inferred (e.g., Fig. 5; subvertical chaotic zones of reflection in seismic data as dikes (Thomson, 2007; Thomson and Hutton, 2004; Rui et al., 2013; Magee et al., 2014; Schofield Wall et al., 2010); and (3) the mapping of magma flow patterns in sills to iden- et al., 2015). The style of flow indicator can also provide insights into mech- tify potential dike feeders (see following). Through the careful application of anisms of emplacement; e.g., intrusive steps occur via brittle fracturing, these techniques and by comparison to field observations, seismic data can whereas magma fingers form through the nonbrittle fluidization of the host be used to delimit magma plumbing systems in more detail than field or other rock (Fig. 4; Pollard et al., 1975; Rickwood, 1990; Hutton, 2009; Schofield et al., geophysical techniques. 2010, 2012a, 2012b).

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A C n

Magma flowB directio C′

A iii.

B′ Sill Figure 4 (on this and following page). Key (A) Magma flow indicators (i.e., intru- Sill sive steps and bridge structures) pro- Intrusion duced by brittle fracture processes. Field Sandstone photos i. and ii. are of intrusive steps observed in inclined sheets and sills on Ardna A′ Shale 5 m murchan, northwest Scotland. Field photos iii., iv., and v. from the Theron Mountains, A A′ i. iv. Antarctica, document the growth of an intact sedimentary bridge between two i. magma segments (iii) into a broken bridge iii. (iv) and a bridge xenolith (v) with increasing Sill distance from the magma source (photos courtesy of Donny Hutton).

B B′ Sill ii. iv. 20 cm ~10 m ii. v.

Sill C C′

v. Sill

50 cm ~3 m

Magma flow patterns mapped across entire sill complexes commonly re- GEOLOGIC EXAMPLES OF SILL COMPLEXES veal that sills are fed by underlying sills or inclined sheets (e.g., Fig. 5B; Thom- son and Hutton, 2004; Magee et al., 2014; Schofield et al., 2015). The extensive Having established how plumbing systems and magma flow patterns can distribution of sill-sill junctions, which are a proxy for feeder relationships, im- be delimited in seismic data and in the field, we here present several field- and plies that interconnected sills form efficient magma migration pathways (Cart- seismic-based case studies of sill complexes (Fig. 2; Table 1). Each case study wright and Hansen, 2006; Muirhead et al., 2012; Magee et al., 2014; Schofield provides important insights into the controls on and impacts of lateral magma et al., 2015). While sill-sill junctions may form via sill-sill abutment or crosscut- flow in sill complexes. To provide context for the seismic-based analyses, we ting (Hansen et al., 2004; Galerne et al., 2011), as opposed to a feeding relation- first review insights into tectonic-magmatic processes obtained from field ship, the mapping of flow patterns can help to constrain intrusion connectivity studies focusing on the Raton Basin in Colorado and the Karoo-Ferrar large ig- within sill complexes. neous province (LIP). We then examine plumbing systems imaged in seismic

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Sill

Figure 4 (continued). (B) Uninterpreted ~30 m ~30 m and interpreted field photo of a sill in Axel Heiburg, Canada, which displays D intrusive steps (black arrows) at a seis C Y X t1 Y mically resolvable scale (courtesy of Mar- tin Jackson). (C) Magma finger formation (modified from Schofield et al., 2012a). (D) Magma fingers in the Golden Valley X Sill, South Africa: top—un-interpreted; bottom—interpreted. t2 X Y ~30 m

t3 X Y

~30 m

data. In particular, we focus on sill complexes affiliated with the North Atlan- with host-rock laminations within the coal becoming more chaotic toward tic Igneous Province that are imaged in seismic data from the Rockall Basin, the intrusion (e.g., Figs. 6A, 6B; Schofield et al., 2012a). Xenoliths of coal the Faroe-Shetland Basin, and the Møre and Vøring Basins (Fig. 2). We also are incorporated into the sill margins, and clasts of basalt occurring within review seismic-based studies of sill complexes from offshore and onshore the coal are interpreted as a peperitic texture (Figs. 6A, 6B; Schofield et al., Australia. 2012a). Occasionally, adjacent magma segments are interconnected by thin (a few tens of centimeters) basalt sheets (e.g., Fig. 6A). Regardless of the variation in sill thickness observed across these segments, it is apparent Raton Basin, Colorado, USA that overlying and underlying sandstone beds remain undeformed (e.g., Fig. 6A; Jackson et al., 2013). This continuity in the strike and dip of sandstone The Raton Basin (Fig. 2) represents a foreland basin that formed during beds implies that sill emplacement was not accommodated by roof uplift the Laramide orogeny. The basin hosts a series of Oligocene–Miocene ba- or floor subsidence (Jackson et al., 2013). Instead, the convoluted bedding saltic sills emplaced within Late Cretaceous–early Paleogene medium-grade within the coal, the lobate coal-sill margins, and the development of peperitic coal beds (Cooper et al., 2007). In cross section, the sills that intruded coal textures all suggest that emplacement occurred via fluidization of the host beds appear to consist of elliptical, finger-like magma segments that are as rock (Schofield et al., 2012a). The intruding basaltic magma is considered much as ~3 m thick, <5 m wide, and may be laterally separated by 5 m (Fig. to have heated the coal until it behaved as a viscous fluid (Schofield et al., 6A). The contacts between the coal and basalt are convoluted and lobate, 2012a), the plastic deformation of which accommodated the intruded magma ≲

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N Karoo-Ferrar LIP A Key Seismically resolved sill outline 1 km Primary lobe Magmatic activity in the Karoo-Ferrar LIP initiated ca. 183 Ma (Encarnación Secondary lobe et al., 1996), in response to plume and rift-related magmatism (Elliot and Flem- Magma finger ing, 2000; Leat, 2008; Hole, 2015). The earliest stages of activity involved rapid Flow direction emplacement (~1 m.y.) of significant magma volumes (>1 × 106 km3) in the geo- Potential sill feeder zone chemically affiliated, yet geographically separated, sill complexes of the Karoo and Ferrar magmatic provinces (Heimann et al., 1994; Elliot and Fleming, 2000; Leat, 2008; Svensen et al., 2012). One of these sill complexes (>3 × 106 km2) intruded into the Karoo Basin, a foreland basin currently exposed onshore South Africa (Figs. 2, 7A, and 7B; Chevallier and Woodford, 1999; Svensen et al., 2012; Scheiber-Enslin et al., 2014). Sills of the Ferrar magmatic province crop out in N Antarctica, southeast Australia, and New Zealand (Figs. 2, 7B, and 7C; Leat, 1 km 2008). Despite the broad geographic distribution of the Ferrar magmatic prov- – B B B′ Key ince, the remarkably homogeneous compositions exhibited by intrusive and Magma flow indicator long axis Amplitude extrusive Ferrar rocks for thousands of kilometers is suggestive of a single L Lobe-lobe contact magma source (Elliot et al., 1999). Ferrar intrusive rocks observed in Antarctica Inferred magma flow direction are found almost exclusively within the 2.5-km-thick Devonian–Jurassic Bea-

+ con Supergroup (e.g., Fig. 7C; Leat, 2008). Paleogeographic reconstructions A A′ Potential sill-sill feeder zone suggest that the Ferrar sill complex extends for more than 4100 km from the N A A′

0.5 km 0.1 s proposed site of a plume head source (Fig. 7B; Elliot et al., 1999; Elliot and B VE ≈ 2 Fleming, 2000; Leat, 2008). Both the Karoo Basin and the basin-filling Beacon 1 km Supergroup rocks, which are relatively flat lying and unfaulted, are dominated by subhorizontal sandstone, shale, and coal beds (e.g., Fig. 7C; Barrett, 1981; Leat, 2008; Svensen et al., 2012). Within the Karoo and Ferrar sill complexes, the abundance of dikes is limited and the absence of extensive (>10 km long) dike swarms suggests that B′ 1 km magma transport primarily occurred and linked to the surface through inter- VE ≈ 3 L connected sill complexes (e.g., Figs. 7A, 7B; Galerne et al., 2008; Leat, 2008;

0.2 s Top Cretaceous Polteau et al., 2008b; Airoldi et al., 2012; Muirhead et al., 2012, 2014; Svensen L L L et al., 2012; Scheiber-Enslin et al., 2014). In the Ferrar magmatic province spe- cifically, the role of sills in facilitating extensive lateral magma flow is sup- Figure 5. (A) Line drawing of magma lobes and fingers imaged in seismic data and used to de- ported by: (1) observed sill-sill feeding relationships (Muirhead et al., 2012, limit sill feeder locations (redrawn from Thomson and Hutton, 2004). (B) Magma flow patterns, 2014); (2) decreasing Mg# and MgO contents along the length of the Ferrar derived from seismically resolved lobe-lobe contacts, consistent with a sill-sill feeding relationship (modified from Magee et al., 2014). VE—vertical exaggeration. magmatic province that are consistent with fractional crystallization during lateral magma flow as far as 4100 km from the Weddell Sea region (Elliot et al., 1999; Elliot and Fleming, 2000; Leat, 2008); and (3) the consistent orientation volume (Jackson et al., 2013). Because coal has a greater viscosity than basal- of bridge and/or broken bridge structure long axes and magnetic lineations tic magma (i.e., there is a high viscosity contrast), the fluid-fluid relationship within sills and interconnected sheets across Antarctica (e.g., in the Theron between the two promoted the formation of magma fingers (Pollard et al., Mountains, Antarctica) (Dragoni et al., 1997; Hutton, 2009; Airoldi et al., 2012). 1975; Schofield et al., 2012a), similar to what is observed during mafic-felsic Overall, these data imply that sills facilitated magma transport lengthwise magma interaction (e.g., Perugini and Poli, 2005). The magma fingers pro- across the East Antarctic margin within Beacon Supergroup rocks (Storey duced by intrusion into coal in the Raton Basin have an elongate geometry and Kyle, 1997; Leat, 2008). It is proposed that the development of a broad (Fig. 6C) and extend for kilometers to tens of kilometers. These observations region of topographic uplift above the proposed plume head generated suffi- highlight that nonbrittle emplacement along compliant horizons can facilitate cient hydraulic head to promote the lateral emplacement of the Ferrar sills for lateral magma transport, without any roof uplift, over considerable distances ~4100 km (Leat, 2008). A similar plume-related mechanism was suggested to (Fig. 6C). explain the lateral (250 km) emplacement of the 2.11 Ga Griffin gabbro sills in

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TABLE 1. MAFIC SILL COMPLEXES: CHARACTERISTICS OF CASE STUDIES Faults Transgressive Distance of lateral Likely Data play a height Area Volume magma transport melt Main sill complex location source Age Host rock lithology key role (km) (km2) (km3) (km) source Key references Raton Basin, Colorado, USA Field Oligocene–Miocene Coal No ???>10 Rift related? Schoﬁeld et al. (2012a) Karoo Basin, South Africa Field Early Jurassic Sandstone, No 3 (?) ~3,000,000>340,000>200 Plume Svensen et al. (2012) shale, and coal Antarctica (i.e., Ferrar sill Field Early Jurassic Sandstone, No ?~560,000<125,000<4100 Plume Leat (2008); Airoldi et al. (2011); complex) shale, and coal Muirhead et al. (2012, 2014) Irish Rockall Basin, offshore Seismic Paleocene–Eocene Shale and No ~4 >748 ?>60 Plume Magee et al. (2014) west Ireland volcaniclastic sandstones North Rockall Basin, offshore Seismic Paleocene–Eocene Shale Yes>3??50–60 Plume Thomson and Hutton (2004); northwest Scotland this study Faroe-Shetland Basin, offshore Seismic Paleocene–Eocene Shale Yes>3>22,500 ?20 Plume Schoﬁeld et al. (2015) east Faroe Islands Møre and Vøring Basins, Seismic Paleocene–Eocene Shale and Yes12>80,000 ??Plume Skogseid et al. (1992); offshore west Norway crystalline basement Plankeetal. (2005); Cartwright and Hansen, (2006) Ceduna subbasin, offshore Seismic Eocene Sandstone, No <1.5 <20,000 ??Hotspot? Jackson et al. (2013); South Australia shale, and coal Mageeetal. (2013b) Northwest Australian Shelf, Seismic Late Jurassic–Early Shale Yes<15 ?? ? Rift related? Symonds et al. (1998); offshore northwest Australia Cretaceous Mageeetal. (2013a, 2013c); Rohrman(2013, 2015) Yilgarn craton Field Proterozoic Granite and Yes<10 ?? >300 Plume Wingate et al. (2008); granitic gneisses Ivanicetal. (2013) ?—parameters that are unknown and/or poorly constrained.

the Hurwitz Basin, Canada (Aspler et al., 2002). The scale of lateral transport in Irish Rockall Basin, North Atlantic Ferrar sills (i.e., as much as 4100 km) rivals that of the ~2000-km-long, 1270 Ma Mackenzie dike swarm of Canada (Ernst and Baragar, 1992). A sill complex located along the northeastern margin of the Irish Rockall Basin consisted of 82 seismically resolved intrusions, and was analyzed in Magee et al. (2014) (Figs. 2 and 8). Individual intrusions predominantly dis- Sill Complexes in the North Atlantic Igneous Province played saucer-shaped and inclined sheet morphologies, which range in diameter from 0.3 to 7.1 km and transgress as much as 1.79 km of strata (Magee The North Atlantic Igneous Province formed ca. 62–53 Ma in response to et al., 2014). The sills are vertically stacked (i.e., numerous sills separated by the break-up of the North Atlantic and the impingement of a mantle plume layers of host-rock strata occur within a vertical sequence) and commonly at the base of the lithosphere (White and McKenzie, 1989; Saunders et al., overlap laterally (Fig. 8B; Magee et al., 2014). The sill complex crosscuts an 1997; Kent and Fitton, 2000; Emeleus and Bell, 2005; Storey et al., 2007; ~2-km-thick stratigraphic sequence of Cretaceous marine shale and Paleo- Hansen et al., 2009). Age relationships and radiometric dating suggest that gene volcaniclastic sandstone, and extends across an area >748 km2 (Figs. the North Atlantic Igneous Province can be divided into two main phases 8A, 8B; Magee et al., 2014). The main inclined portions of the intrusions pri- of magmatic activity; one between ca. 62 and 58 Ma (e.g., west Greenland, marily dip to the northwest and the sills appear to be interconnected; i.e., Great Britain) and the other (e.g., east Greenland) spanning ca. 57–53 Ma inclined sheets and sill limbs apparently feed stratigraphically saucer-shaped (Saunders et al., 1997). Voluminous sill complexes, dike swarms, volcanic sills (Figs. 5B and 8A; Magee et al., 2014). Because acoustic energy is attenu- centers, and extrusive igneous rocks, primarily of basaltic composition, ated within igneous rocks, seismic imaging directly beneath sills is commonly were emplaced during these phases (~6.6 × 106 km2 in total) and are located poor and it is thus difficult to confirm whether inferred sill-sill junctions are along two volcanic rifted margins; the conjugate east Greenland–northwest physically connected (e.g., Fig. 5B; e.g., Smallwood and Maresh, 2002; Cart- European and the west Greenland–Baffin Bay margins (Fig. 2; Storey et al., wright and Hansen, 2006). However, a comparison between the Irish Rockall 2007; Hansen et al., 2009). Basin seismic data and synthetic seismic forward models, designed to test

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A Sandstone

Sill

Fig. 6B

2.5 m

B C Figure 6. (A) Field photo of magma fingers Magma fingers developed within a coal layer in the Raton Basin, Colorado (USA). Note that the over- No uplift of sandstone lying sandstone bed displays no evidence of uplift above the thicker portions of the magma fingers. (B) Closeup highlighting the convoluted coal laminations close to the intrusion and the clasts of dolerite within the coal (A and B are modified from Convoluted Schofield et al., 2012a). (C) Conceptual box model of magma fingers in the Raton 1.5 m laminations in coal Basin.

Sill

1.5 m Not to scale

how sill connectivity affects reflection configurations, suggests that at least of dome-shaped folds are developed within the upper Cretaceous to top some sills are physically connected, with lower sills feeding overlying sills lower Eocene strata (Fig. 8A). The outline of these folds corresponds to the (Magee et al., 2015, Fig. 11 therein). Connectivity between individual sills is lateral termination of underlying sills and, particularly along the top Paleo further supported by mapping of flow indicators, which commonly radiate cene horizon, seismic reflections can be observed to onlap onto the folds outward from inferred sill-sill feeder zones (e.g., Fig. 5B; Magee et al., 2014). (Fig. 8A). These seismic-stratigraphic relationships imply that fold growth Observations related to sill connectivity are consistent with a regional south- was generated by and accommodated sill emplacement, resulting in the for- east-directed lateral magma flow regime, perhaps indicative of a melt source mation of bathymetric highs that were onlapped by synkinematic sediment associated with the Hebridean Terrace igneous complex located ~10 km to the (Fig. 8C; Trude et al., 2003; Hansen and Cartwright, 2006b; Jackson et al., northwest (Magee et al., 2014). Based on the position of the furthest sills from 2013; Magee et al., 2013a, 2014). Because the sills are vertically stacked and the Hebridean Terrace igneous complex, it is plausible that this sill complex laterally overlap each other, the folds display a complex compound fold mor- documents at least 60 km of lateral magma flow. phology (Fig. 8A); i.e., during the growth of individual forced folds, continued In addition to delimiting plumbing systems and flow patterns within sedi- magma intrusion promoted fold coalescence and formed folds as much as mentary basins, it is also important to constrain the time frame of magmatic 244 km2 and 296 km high (Fig. 8C; Magee et al., 2014). Onlap relationships are activity. This is because the rate of intrusion and volume of melt can pro- observed throughout the folded succession, between the top Cretaceous and vide important insights into (1) the evolution of the melt source (White et al., top lower Eocene boundaries (Fig. 8A), suggesting that sill emplacement 2008); (2) the longevity of potential volcanic activity; and (3) deformation of and fold growth occurred incrementally over 15 m.y. between ca. 65 and the host rock. Above the sill complex mapped by Magee et al. (2014), a series 54 Ma (Magee et al., 2014).

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A’ South A 28°E Namibia B Karoo ~1000 km America Africa Basin

Lesotho Mad. A India

32°E

Karoo Basin Antarctica

South Africa

Figure 7. (A) Distribution of sills (black) GONDWANA within the Karoo Basin (dashed outline) and a schematic cross section (modified 20°E 24°E 28°E Tasmania ~180 Ma from Svensen et al., 2012). (B) Paleo- A A′ km geographic reconstruction (ca. 180 Ma) 2 New Zealand highlighting the current extent of the Australia Karoo-Ferrar large igneous province and 0 Current Beacon Supergroup (+ correlatives) associated basin exposures (based on Ferrar magmatic province Karoo magmatic province Mortimer et al., 1995; Leat, 2008; Hastie –2 250 km et al., 2014). Mad.—Madagascar. (C) Field Proposed plume head positions Dike swarms photo and interpreted photo of inter C connected Ferrar sills and inclined sheets 30 m 30 m intruded into the Beacon Supergroup in Antarctica (courtesy of James White).

Sills

Inclined sheets

Beacon Supergroup

Northeast Rockall Basin, North Atlantic and are situated along major crustal lineaments, which likely provided magma ascent pathways connecting the melt source to the Rockall Basin (Archer et al., Similar to the Irish sector of the Rockall Basin, sill complexes within the north- 2005; Hansen and Cartwright, 2006b). Despite the spatial association between east Rockall Basin were emplaced into upper Cretaceous marine shale during sill complexes and igneous centers in the northeast Rockall Basin (e.g., Figs. the Paleocene and Eocene (e.g., Fig. 9A; Thomson and Hutton, 2004; Archer 9A, 9B), any genetic relationship between the two remains poorly constrained. et al., 2005; Hansen and Cartwright, 2006b). Associated with the sill complexes To test whether the sill complexes were fed from the igneous centers, we are series of large (~20 km diameter) igneous centers, which are interpreted examine new observations of magma flow patterns exhibited by a sill complex to consist of broad volcanic edifices fed by underlying laccoliths and plutons located along the eastern margin of the northeast Rockall Basin (Fig. 9). The 2D (e.g., Figs. 9A, 9B; Archer et al., 2005). These igneous centers commonly occur seismic data reveal that the sill complex covers a broad area and extends toward in northwest-southeast– and east-northeast–west-southwest–trending chains the Darwin igneous center, which is located ~60 km to the northwest (Figs. 9A, 9B).

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A S Onlap Truncation Downlap N

Top Lower Eocene

Top Paleocene

Top Cretaceous

VE ≈ 5

0.25 s (TWT) 2 km B 6204000 6212000 6220000 6228000 6236000 N 44600 5 km

Figure 8. (A) Seismic section of forced folds Fig. 8A and the underlying sill complex imaged along the eastern margin of the Irish Rockall Basin. Sills are primarily emplaced 0 Sill base ~ <1 km within the Cretaceous, shale-dominated below top Paleocene

4540 succession. The Paleocene and Eocene Sill base ~ 1–2 km strata are volcaniclastic- and shale-domi- below top Paleocene nated, respectively. VE—vertical exaggera- Sill base ~ >2 km tion. (B) Sill distribution map highlighting below top Paleocene Fig. 5B the overlapping nature of individual sills. (C) Conceptual model of fold growth and C t - Initial intrusion and instantaneous fold growth Key coalescence in response to the incremen- 0 tal injection of magma pulses (t—time) New magma (A–C modified from Magee et al., 2014). pulse volume

t0 magma pulse volume

t1 magma pulse volume Pre-intrusion t1 - Continued intrusion and onset of fold coalescence strata

t1 synfolding strata and onlap

t2 synfolding strata and onlap Group 1b periclinal fold base datum t2 - Increased fold growth accommodating later magma pulses Group 2 compound fold base datum Group 3 compound fold base datum NB: Intrusion volume is to highlight different magma pulses, not the distribution of magma within single sills from each pulse.

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A NW SE )

VE ≈ 5

1 s (TWT 1 km Darwin Igneous Center Northeast Rockall Basin West Lewis High

NW 3D seismic survey SE Figure 9. (A) Regional two-dimensional seismic line highlighting the distribution Seabed of sills within the northeast Rockall Basin. See Figure 10B for location. TWT—two- way traveltime; VE—vertical exaggeration; 3D—three dimensional. (B) Simplified Top Paleogene geological map of the study area. Local igneous centers: RB—Rosebank; D—Dar- win; SS—Sula Sgeir; G—Geikie; SK— Sill-complex Saint Kilda. The Darwin-Geikie High (DGH)

) and the West Lewis High (WLH) are also shown. The study of Thomson and Hutton (2004) examined magma flow patterns in VE ≈ 5 sills located just to the southwest of the

1 s (TWT 1 km location of the 3D seismic data used here. N (C) Sill outlines and mapped magma flow B North-east SS C patterns. A map of an arbitrary seismic Rockall line through one sill is provided to demon- D Basin 2 km strate how magma lobes were used to delimit magma flow patterns. WLH Fig. 9A RB

North DGH 3D seismic survey; Rockall Magma flow direction Basin Fig. 9C N G

Outer Hebrides 50 km SK Rockall Basin A A’ West ) Structural highs N Sill Lewis Igneous centers High Undifferentiated basins and cont. shelf

Major normal faults 10 km 0.1 s (TWT 1 km

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Here we focus on a portion of the sill complex imaged within a 3D seismic data Basin sill complex implies that magma was fed into the basin via basement- set that is located ~50 km southwest of the Sula Sgeir igneous center (Figs. involved faults, from which sills propagated laterally for as much as ~25 km 9A, 9B). Of 100 mapped intrusions, 72% have a saucer-shaped morphology into the hanging wall (Figs. 10A, 10B; Schofield et al., 2015). and the remaining 18% are classified as inclined sheets. The sills appear to be interconnected, with inclined sheets and sill limbs predominantly dipping to the northwest. Mapping of magma lobes and other flow indicators within in- Møre and Vøring Basins, North Atlantic dividual intrusions reveal that magma primarily propagated to the southwest or southeast, consistent with typical northwest-dipping orientation of trans- Sill complexes developed along the Norwegian margin (Fig. 11A) repre- gressive intrusion portions (Figs. 9B, 9C). Similar magma flow patterns were sent a continuation of the Late Cretaceous–Paleogene magmatic activity that recorded by Thomson and Hutton (2004) ~20 km to the southwest of the study generated networks of intrusions within the Rockall and Faroe-Shetland Ba- area (Fig. 9B). Southwest-directed magma flow patterns are particularly well sins to the southwest (Fig. 2; Skogseid et al., 1992; Planke et al., 2005; Cart- developed adjacent to and along strike of a northwest-dipping, basin-bound- wright and Hansen, 2006; Hansen, 2006). Sills were predominantly emplaced ing fault, perhaps suggesting that sill propagation to the southeast was inhib- into Late Cretaceous and early Paleocene shale, although some sills intruded ited by the juxtaposition of sedimentary strata against crystalline basement in older sedimentary strata and crystalline basement (e.g., Cartwright and Han- the fault footwall (Fig. 9B). These observations imply that the mapped sill com- sen, 2006). A close spatial relationship between the lateral tips of sills and an plex could have been fed from the Darwin and/or Sula Sgeir igneous centers overlying complex of 734 seismically resolved, genetically related hydrother- via lateral magma flow over ~50–60 km (Figs. 9A, 9B). Geochemical and further mal vents suggests that (Fig. 11A) (1) the stratigraphic level of extrusion rep- seismic analyses are required to test this hypothesis. resents the paleoseabed contemporaneous to sill intrusion; and (2) emplacement occurred ca. 55 Ma (Svensen et al., 2004; Planke et al., 2005; Hansen, 2006). Overall, the sill complexes, which appear to be internally interconnected Faroe-Shetland Basin, North Atlantic via sill-sill junctions, extend across >80,000 km2 of the basin and, in places, transgress as much as 12 km of sedimentary strata and crystalline basement An extensive sill complex covering >22,500 km2 and emplaced primarily (e.g., Figs. 11A–11C; Svensen et al., 2004; Planke et al., 2005; Cartwright and within upper Cretaceous shale is imaged in seismic reflection data from the Hansen, 2006). Some of the sills that extend into basement rocks may link to a Faroe-Shetland Basin (Figs. 10A, 10B; Passey and Hitchen, 2011; Schofield high-velocity body inferred to represent the melt source (Fig. 11C; Cartwright et al., 2015). Radiometric dating of sills penetrated by boreholes and relative and Hansen, 2006). Individually, the majority of sills display saucer-shaped and ages of sills determined from seismic-stratigraphic onlap relationships of inclined sheet geometries (e.g., Fig. 11B), regardless of whether they intruded overlying strata onto intrusion-induced forced folds suggests that intrusion sedimentary or crystalline rocks (Planke et al., 2005; Cartwright and Hansen, occurred between ca. 61 and 52 Ma (Passey and Hitchen, 2011; Schofield et al., 2006). Any major variations in sill geometry are primarily displayed by those 2015). While >300 seismically resolved sills have been mapped in the basin intrusions emplaced at depths <1 km beneath the paleoseabed; these shal- (Fig. 10A), the actual number of intrusions that constitute the sill complex is low-level sills are lobate in form and display prominent magma flow channels likely much larger because:(1) imaging beneath the Paleogene flood basalt (Fig. 11D; Hansen and Cartwright, 2006a; Miles and Cartwright, 2010). Obser- cover is poor, compromising mapping of underlying intrusions; and (2) bore- vations from the Møre and Vøring Basins are consistent with those gathered hole data suggest that a significant proportion (as much as ~80%) of intrusions from the Rockall and Faroe-Shetland Basins; i.e., interconnected sill complexes have thicknesses below the limit of detectability of the seismic data (Schofield within sedimentary basins are laterally extensive and also provide an efficient et al., 2015). The sills that are resolved display a wide variety of morphologies mechanism for magma ascent (e.g., Fig. 11B; Cartwright and Hansen, 2006). (e.g., saucer-shaped sills and inclined sheets) and have diameters of as much as tens of kilometers (e.g., Figs. 10A, 10B; Schofield et al., 2015). Individual intrusions may transgress as much as 2 km of the sedimentary succession and Ceduna Subbasin, Offshore Southern Australia appear to connect to overlying sills, inclined sheets, and lava flows (e.g., Figs. 10A, 10B; Schofield et al., 2012b, 2015). Some inclined portions of intrusions An array of 46 intracontinental volcanoes and underlying sills and lacco- coincide with and likely intruded along fault planes (Schofield et al., 2015). liths are imaged in seismic data from the Ceduna subbasin (Fig. 12; Jackson, Connectivity between sills is supported by mapped flow indicators (Schofield 2012; Magee et al., 2013b). Dredged samples from volcano summits exposed et al., 2015). For example, the long axes of bridge and broken bridge structures, at the seabed in the west of the Ceduna subbasin suggest that the volcanoes which bound magma lobes in the Flett Ridge Sill, radiate out from the inferred have a basaltic composition (Fig. 12A; Clarke and Alley, 1993). The edifices magma input zone at a sill-inclined sheet junction (Figs. 10C, 10D; Schofield range in height from 0.02 to 1 km, have diameters as much as ~18 km, and et al., 2012b). The reconstruction of flow patterns within the Faroe-Shetland flank dips are <12°, indicative of a shield volcano morphology (e.g., Fig. 12;

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A Faroe Islands 5°W 3°W C 62°N

Fig. 10A SBF Key to (A) SB Shetland Islands Structural high Basin (iii) A B Scotland Fault (tick denotes A’ FLB downthrown side) Lineament or transfer zone (ii) A B C Faroe-Shetland FBF Basin limit Main zones of (i) A B C 61°N magma input FBF Magma flow direction Sill Stratigraphic height – + Figure 10. (A) Regional geology of the Source 100 km Shetland Faroe-Shetland Basin, including seismi- zone cally mapped sills and magma flow direc- NW 214/4-1 214/9-1 208/21-1 SE B 1 km tions. Magma flow patterns were used to (i) infer magma input zones into the basin (modified from Schofield et al., 2015). ) ABC (B) Representative seismic line (uninterpreted and interpreted) highlighting sill complex structure and association with faults (Schofield et al., 2015). TWT—two-

iii)0.3 s (TWT 0.7 km way traveltime. (C) Opacity render of the (ii) Flett Basin Sill and corresponding seismic

) sections documenting intrusive step and A B C broken bridge growth along lobe-lobe

1 s (TWT contacts (modified from Schofield et al., 2012b). A, A’, B, and C correspond to intru-

0.3 s (TWT 0.7 km sive steps or bridge structures observed

) (iii) in profiles (i), (ii), and (iii). (D) Geobody

20 km ) interpretation of the Flett Basin Sill show- A ing that the magma lobes imaged in C NW 214/4-1 214/9-1 208/21-1 SE A’ B trace back to, and were likely fed from, a fault-hosted inclined sheet (modified from 0.3 s (TWT 0.7 km Schofield et al., 2012b). D Fig. 10C view

Saucer-shaped sill 3.7

1 s (TWT Flett Ridge TWT Sill

4 (s) ) 20 km Key for (B) Subaerial lavas and hyaloclastites Intrusions Paleogene Magma flow direction 4.4 Main zones of magma input into basin fill 3 km Inclined sheet feeder Cretaceous Fault

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0° 3°E 6°E 9°E of the magmatism in the Ceduna subbasin remains enigmatic. Recent studies, A C Seabed Sill Vøring Marginal Fig. 11C however, favor a model whereby melting was associated with a static, intra- 67°N High 3 Paleoseabed continental hotspot located directly beneath the volcanic field (Schofield et al., Vøring 2008). Although magmatism was not linked to rifting, this case study presents (s) Basin a unique opportunity to examine the relationship between volcano growth and Fig. 11D TWT the subvolcanic plumbing system. 5 65°N By analyzing the external morphology of the volcanoes and assessing the Sill-complex Møre extent configuration of intravolcano seismic reflections, it was demonstrated (Magee Marginal T-reflection et al., 2013b) that the basal diameter and summit height proportionally in- High 7 Fig. 11B High-velocity body 5 km creased through progressive summit eruptions (Figs. 12C, 12D). Mapping of the volcanoes and associated intrusions reveals that 40% of the volcanoes 63°N Møre Basin D N overlie the lateral terminations of sills and/or laccoliths; 30% of the volcanoes are located directly above or close to upper fault tips, whereas the remain- 1 km ing 30% show no spatial relationship to underlying intrusions or faults (Figs. 12A, 12B; Magee et al., 2013b). The intrusions occur within the Potoroo Forma- Seabed B tion, which consists of interbedded fluvial sandstones and floodplain shales Paleoseabed and coals, and were emplaced at depths of as much as 1.5 km beneath the paleosurface (i.e., the Pidinga Formation) (Jackson et al., 2013). It is difficult 3.5 to assess the sill-sill and sill-volcano connectivity due to the ≥4 km spacing of Key the 2D seismic lines. However, if we assume that the sills and laccoliths are

TW T (s ) interconnected, similar to many sill complexes worldwide that display similar Seismically 4.5 resolved morphological characteristics (this study), a laterally extensive plumbing sys- sill outline tem within the Ceduna subbasin would imply that the melt source may not be Lobe located directly beneath the volcanoes (Magee et al., 2013b). Such an interpre- Inferred flow direction tation of a melt source laterally offset from the volcano array contrasts with the 5.5 Potential sill traditional assumption that intracontinental hotspot basaltic shield volcanoes 2 km feeder zone are fed from an underlying zone of melting via dikes (Valentine and Gregg,

Figure 11. (A) Sill complex distribution (gray) within the Møre and Vøring Basins, offshore Nor- 2008). Alternatively, dikes not imaged in these seismic data may feed the shal- way (modified from Svensen et al., 2004). (B, C) Line drawings of two different seismic lines low-level intrusions. A role for dikes within the plumbing system of the Ceduna from Cartwright and Hansen (2006). The T-reflection corresponds to the top of the high-velocity subbasin is supported by the observation that some volcanoes (30%) are not body. TWT—two-way traveltime. (D) Mapped magma lobes and inferred flow directions for the Solsikke sill are shown (redrawn from Hansen and Cartwright, 2006a). associated with underlying sills or faults (Figs. 12A, 12B; Magee et al., 2013b). Analyses of intrusion thickness (i.e., where discrete upper and lower contact reflections are resolved) and forced fold amplitude allow mecha- Magee et al., 2013b, their GP1 volcanogenic mounds). The volcanoes are on nisms of sill emplacement to be assessed (Hansen and Cartwright, 2006b; the top of the Pidinga Formation, are onlapped by the Nullarbor Limestone for- Jackson et al., 2013; Magee et al., 2013a). If it is assumed that only roof up- mation, and several are situated directly above the upper tips of normal faults lift accommodates the intruded magma volume, then fold amplitude should (Fig. 12B; Jackson, 2012; Magee et al., 2013b). These seismic-stratigraphic ob- equal the intrusion thickness (Hansen and Cartwright, 2006b). Discrete upper servations and fault relationships are consistent with an emplacement age of and lower contact reflections for individual intrusion can occasionally be 42 Ma (Jackson, 2012; Magee et al., 2013b). A series of intrusion-induced forced mapped within the Ceduna subbasin, allowing the relationship between in- folds developed above sills and laccoliths are onlapped by the volcanoes and trusion thickness and fold amplitude to be investigated (Jackson et al., 2013). the Nullarbor Limestone (Fig. 12B; Jackson et al., 2013; Magee et al., 2013b). Some of the sills and folds have a 1:1 relationship between intrusion thick- This implies that sill and volcano emplacement were contemporaneous and ness and fold amplitude, implying that roof uplift accommodated intrusion likely genetically related (Jackson, 2012; Magee et al., 2013b). Magmatic activ- (Fig. 13; Jackson et al., 2013). However, the majority of the fold amplitudes ity thus occurred immediately after the gravitational collapse of the southern measured are as much as 83% less than (average of 37%) the thickness of Australian continental margin, ~40 m.y. after continental break-up between underlying intrusions (Fig. 13; Jackson et al., 2013). Similar discrepancies be- Australia and Antarctica (Schofield et al., 2008). Due to the lack of a temporal tween sill thickness and fold amplitudes are observed elsewhere in other sedi relationship between volcano emplacement and continental rifting, the origin mentary basins (Fig. 13; Hansen and Cartwright, 2006b; Magee et al., 2013a).

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A 45 Ma Australia

60°S N

Fig. 12B 70°S Antarctica 500 km Continental crust Mid-ocean ridge Thinned continental crust Dredge site Oceanic crust 10 km Figure 12. (A) Paleogeographic reconstruc- Volcano summit Normal fault Top Pidinga Fmn depth (km) Volcano thickness (km) tion (modified from Espurt et al., 2009) B Sill outline 0.9 2.6 0.8 0 documenting the regional structure immediately prior to the emplacement of a NW SE suite of volcanoes mapped in the Ceduna Seabed subbasin (inset) (modified from Magee Shale-dominated v v et al., 2013b). (B) Seismic section delimit- v v ing high-amplitude reflections (s—sills), ff ff forced folds (ff), and volcanoes (v) (modi- Nullarbor Limestone Fmn ff ff fied from Jackson et al., 2013). Fmn—formation; TWT—two-way traveltime; VE— Sandstone-dominated vertical exaggeration. (C) Plot of volcano (shale and coal) s basal diameter against summit height Fig. 12D s Top Potoroo Fmn Top Pidinga Fmn (Magee et al., 2013b). (D) Internal reflec- s s tions within a volcano that are inferred to s Fig. 3D relate to discrete tuff layers and/or lava flows. s

VE ≈ 10

0.4 s (TWT) 10 km C D NW SE Seabed 0.8 )

0.6 Nullarbor Limestone Fmn Internal layers 0.4 Top volcano

0.2 0.2 s (TWT Summit height (km 0 0 5 10 15 20 Base volcano Basal diameter (km) VE ≈ 5 1 km )

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Key Strata-concordant sill

Saucer-shaped sill Jackson et al. (2013) variants 300 Laccoliths

Sill 40 Hansen and 250 Sill 41 Cartwright (2006b)

) Magee et al. (2013a) 1 Saucer-shaped sill f/t = (m

) 200 Figure 13. Plot of intrusion thickness (mea- (f sured where upper and lower intrusive 2 5

ud e contacts can be distinguished) against the f/t = 0. it 150 f/t = amplitude of overlying forced folds. pl am

33 100 f/t = 0.

Fo ld 5 f/t = 0.2

0 050 100 150 200 250 300 350 Intrusion thickness (t) (m)

This discrepancy between sill thickness and fold amplitude could perhaps be McClay et al., 2013; Rohrman, 2013). These emplacement ages, constrained attributed to measurement errors (e.g., incorrect depth conversion) or geologic from onlap of strata onto intrusion-induced forced folds and the presence processes that restrict the maximum fold amplitude (e.g., out-of-plane strain of lava flows emplaced along the top Aptian, indicate that magmatic activity or strain interference between neighboring folds; Jackson et al., 2013). In Jack- occurred immediately prior to and during continental break-up (Magee et al., son et al. (2013) it was suggested that other space-making mechanisms may 2013a; McClay et al., 2013; Rohrman, 2015). A key observation is that inclined play a role during emplacement and thus control the intrusion thickness to sheets and sill limbs commonly coincide with and likely intruded along fault fold amplitude ratio. This is supported by numerous field- and seismic-based planes for as much as 1 s two-way traveltime (e.g., ~300 m; Fig. 14A; Magee observations (Schofield et al., 2010, 2012a; Jackson et al., 2013; Magee et al., et al., 2013c; McClay et al., 2013). However, in Magee et al. (2013c) it was noted 2013a). For example, sills intruded into coal in the Raton Basin were emplaced that only certain sill portions appear to intrude up fault planes, with other via the development of a viscous fluid-fluid contact between the magma and sections crosscutting faults (e.g., Fig. 14A). Field observations and mapping host rock; i.e., space for the magma was generated by plastic deformation and of flow indicators in seismic data suggest that the interaction style between porosity reduction of the coal (e.g., Fig. 6; Schofield et al., 2012a). Space-mak- magma and faults is primarily controlled by the fault-plane geometry and ori- ing mechanisms that do not involve roof uplift may also occur during intrusion entation (Valentine and Krogh, 2006; Bedard et al., 2012; Magee et al., 2013c). In into siliciclastic sequences (e.g., sandstone compaction, Morgan et al., 2008; particular, sills may only intrude up faults when they are hosted in the hanging shale fluidization, Schofield et al., 2010). The likely occurrence of multiple pro- wall and flowing toward or along the strike of the fault plane; i.e., uplift of the cesses accommodating intrusion implies that overburden uplift, and therefore overburden reduces the normal stress across the fault, allowing magma to the degree of surface deformation, may not directly correlate to the volume or intrude (Fig. 14B; Magee et al., 2013c). location of magma emplaced (Magee et al., 2013a). Northern Yilgarn Craton, Western Australia Northwest Australian Shelf The Yilgarn craton in Western Australia (Fig. 15A) consists of metavol canic The Paleozoic–Holocene, ~535,000 km2 North Carnarvon Basin, located off- and metasedimentary rocks, granites, and granitic gneisses that formed prin- shore northwest Australia (Fig. 2), contains numerous Mesozoic depocenters cipally between ca. 3.05 and ca. 2.62 Ga (Cassidy et al., 2006). The Yilgarn (e.g., the Exmouth subbasin) that host sill complexes emplaced between the craton is subdivided into a series of terranes, including the Youanmi terrane, Kimmeridgian and Aptian (Symonds et al., 1998; Magee et al., 2013a, 2013c; which contains substantial greenstone belts separated by granite and granitic

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F1 F4 F5 F3 F7 F6

F5 F6 F7 F1 North survey limit F3 F4 Figure 14. (A) Three seismic sections high- VE ≈ 5 light different magma-fault interaction styles. In places the sill crosscuts the fault, 200 ms (TWT ) 1 km in others it steps up the fault, and occasionally its inclined limb coincides with F2 and likely intruded along the fault plane for ~300 ms two-way traveltime (TWT; F3 F4 F5 F1 VE—vertical exaggeration). The sills are emplaced into Late Jurassic mudstones F7 (Magee et al., 2013c). The sill map shows F2 F6 that the sill extends up faults when its + Fault magma flow direction in the hanging wall N is parallel to fault strike or toward the fault TWT) Sill intruded plane. (B) Schematic diagram demonstrat- along fault ing how uplift related to magma intruding

VE ≈ 5 me (s Magma flow direction toward a fault in the hanging wall may

Ti – 200 ms (TWT ) 1 km 1 km serve to open the fault (i.e., a decrease in Hanging wall σn) and vice versa (i.e., an increase in σn). B σv σv Modified from Magee et al. (2013c). σv Footwall F4 F5 F2 F3 σn Sill Magma flow direction F6 Footwall Hanging wall ) σn

σv σv σv VE ≈ 5 Sill

200 ms (TWT 1 km

gneiss (Fig. 15A; Wyche et al., 2013). In 2010, 3 deep seismic reflection pro- The seismic data were acquired in a region transected by numerous files totaling 694 km in length were acquired to 20 s two-way traveltime in the Proterozoic mafic dikes and sills associated with the Widgiemooltha (ca. northern Youanmi terrane (Fig. 15B; Costelloe and Jones, 2013). The interpreta- 2410 Ma), Marnda Moorn (ca. 1210 Ma), and Warakurna (ca. 1070 Ma) LIPs tion of these profiles has led to the identification of an interconnected network (Fig. 15B; Wyche et al., 2013). Dolerite dikes are typically 5–30 m wide, 50– of highly reflective, generally saucer-shaped sills that extend for several hun- 300 km long, and trend mainly east to northeast, while sills recognized at the dreds of kilometers (e.g., Figs. 15C, 15D; Ivanic et al., 2013). The sills mainly oc- surface are as much as 50 m thick and 300 km in length (Ivanic et al., 2013). The cur shallower than 3 s two-way traveltime, and typically dip at <10°–20° (Figs. Widgiemooltha and Marnda Moorn LIPs are generally characterized by dike 15C, 15D). Seismic modeling, assuming representative P-wave velocities and suites that are not evident in the seismic data due to their steeper dips (Thom- densities for dolerites and granites, indicates that the sills have a minimum son, 2007), and thus the intrusions identified in the Youanmi seismic lines are thickness of 25 m, and some may be as much as 200 m thick (Ivanic et al., interpreted to belong to the ca. 1070 Ma Warakurna LIP (Ivanic et al., 2013). 2013). Some sills appear to have been intruded along shear zones, while others The Warakurna LIP consists of mafic igneous rocks that were rapidly emplaced crosscut and truncate reflections associated with preexisting shear zones and within an area of ~1.5 × 106 km2 in central and Western Australia, and is in- listric faults (Figs. 15C, 15D; Ivanic et al., 2013). terpreted to have been generated by a mantle plume (Wingate et al., 2004).

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A 112ºE 116ºE 120ºE 124ºE B 100 km Unassigned Proterozoic dikes Mundine Well Dolerite Suite (~755 Ma) 10GA-YU1 Warakurna LIP intrusions 26ºS (~1070 Ma) Marnda Moorn LIP intrusions (~1210 Ma) Widgiemooltha LIP intrusions 10GA-YU10GA-YU3 2 1 0GA- (~2410 Ma) Fig. 15B Y U1 Youanmi 30ºS Yilgarn terrane Craton Perth Proterozoic- Phanerozic rocks Archean granites 1000 km 10GA-YU2 Archean gabbro 10GA-YU3 Figure 15. (A) Geology of the Yilgarn craton Archean supra- showing terrane boundaries and locations crustal rocks of Youanmi seismic lines (modified from 34ºS Wyche et al., 2013). (B) Interpreted geo- Seismic line logical map of the northern Yilgarn craton

200 km 7000 with CDP showing suites of Proterozoic intrusions in the region of the Youanmi seismic lines (modified from Ivanic et al., 2013). LIP— C large igneous province; CDP—common CDP 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 depth point; TWT—two-way traveltime. 0 (C) Interpreted deep seismic reflection profile 10GA-YU1. Note abundant high-amplitude reflection packages interpreted to be saucer-shaped sills in the shallow parts 5 (<5 s two-way traveltime, TWT) of the

T (s) profile (modified from Ivanic et al., 2013). (D) Interpreted migrated deep seismic re- TW 10 flection profile 10GA-YU2 (modified from Moho Ivanic et al., 2013). 10GA-YU1 Upper mantle 25 km 15 Dike or sill Base Cenozoic Base roof zone Base upper crust Form line Granite Base lower zone Base middle crust Fault Base felsic volcanic rocks Base ultramafic zone Top Moho transition zone D Base mafic rocks Moho CDP 5000 6000 7000 8000 9000 1000011000 12000 13000 14000 15000 16000 17000 0

5 T (s)

TW 10 Moho Upper mantle 25 km 15 10GA-YU2

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The sill that extends from common depth point (CDP) 6500 to CDP 11000 on tral buoyancy) and the magma pressure overcomes the strength of the wall seismic profile 10GA-YU2 (Fig. 15D) is thought to be the Mount Homes Gabbro rock and lithostatic load (Gilbert, 1877; Francis, 1982); (2) mechanical contrasts Sill, which crops out near the town of Sandstone and has a crystallization age in material toughness and Young’s modulus across an interface, resulting in of 1070 ± 18 Ma based on sensitive high-resolution ion microprobe U-Pb dating an elastic mismatch that promotes deflection of dikes or inclined sheets into of baddeleyite (Wingate et al., 2008). The exposed portion of the Mount Homes sills (Gudmundsson, 2011, 2014; Barnett and Gudmundsson, 2014); (3) the local Gabbro Sill, which is ascribed to the Warakurna LIP, consists of an ~5-m-thick rotation of the minimum compressive principal stress axis to vertical, favor- medium- to coarse-grained gabbro, dips ~5° to the north, and intrudes meta- ing sill intrusion (Anderson, 1951; Mudge, 1968; Gretener, 1969; Leaman, 1975; morphosed Archean mafic and ultramafic rocks (Wingate et al., 2008). To the Kavanagh et al., 2006; Valentine and Krogh, 2006; Menand, 2008; Menand north of line 10GA-YU2, the sill is exposed again and has a thickness of ~30 m et al., 2010; Kavanagh and Pavier, 2014); and (4) the exploitation of compliant and margins containing abundant granitic xenoliths (Ivanic et al., 2013). Aero- horizons via nonbrittle mechanisms (Pollard et al., 1975; Einsele et al., 1980; magnetic data suggest that the strike length of the sill exceeds 400 km (Win- Schofield et al., 2010, 2012a). The broad depth range across which individual gate et al., 2008). A minimum constraint on the depth of emplacement of these sill complexes extend (e.g., Figs. 6–15) suggests that the level of neutral buoy- sills is provided by apatite fission track data, which indicate that the northern ancy exerts little influence on sill emplacement (e.g., Johnson and Pollard, Yilgarn craton has been exhumed by at least ~3 km since the early Permian 1973; Cartwright and Hansen, 2006; Menand, 2011; Taisne et al., 2011; Gud- (Weber et al., 2005). The interconnected network of saucer-shaped sills inter- mundsson, 2012). We therefore discuss the processes controlling sill emplace- preted from the Yilgarn craton seismic data share many characteristics with ment along rock-rock interfaces (e.g., bedding planes) or compliant horizons those documented in the other case studies presented herein. It is important and examine how space is generated for the intruded magma volume. that the Yilgarn craton data suggest that interconnected sill complexes facilitat- One important control on sill emplacement is elastic mismatch (e.g., Gud- ing long-distance lateral magma transport can also develop within crystalline mundsson, 2011; Barnett and Gudmundsson, 2014). When a propagating verti- continental crust. cal or inclined extension fracture approaches a subhorizontal interface between two layers, across which there is a significant difference in material toughness and Young’s modulus, its continued propagation direction is dependent on (Fig. DISCUSSION 16A; e.g., Gudmundsson, 2011; Barnett and Gudmundsson, 2014): (1) the ratio

of the energy release rate required to penetrate the interface (Gp) or deflect

Seismic reflection data and field observations suggest that upper crustal along it (Gd), which is a function of the material toughness of the interface, the plumbing systems, particularly those emplaced within sedimentary basins, overlying layer, and the stress intensity factor for mode I and mode II cracks; may be dominated by interconnected sills and inclined sheets (e.g., Figs. relative to (2) Dundurs elastic mismatch parameter, which characterizes the dif- 6–15). Two recurring observations of shallow-level plumbing systems are ap- ference in the Young’s modulus between the two layers. Experimental inves- parent from the case studies presented here: (1) sill complexes can provide tigations corroborate that elastic mismatch will typically favor deflection of a efficient magma flow pathways, transporting melt from source to surface over dike or an inclined sheet into a sill if the overlying layer is stiffer than the under- great vertical (tens of kilometers) and lateral distances (more than hundreds of lying layer (Fig. 16A; Kavanagh et al., 2006; Gudmundsson, 2011; Barnett and kilometers) (Table 1; e.g., Cartwright and Hansen, 2006; Leat, 2008; Schofield Gudmundsson, 2014). This effect is further enhanced when magma pressure et al., 2015); and (2) host-rock lithology and structure influence the stratigraphic exceeds the fracture toughness of the interface (Kavanagh and Pavier, 2014), or level of emplacement and how an intruded magma volume is accommodated magma flux and temperature conditions, which control solidification, favor sill (e.g., Schofield et al., 2012a, 2014; Jackson et al., 2013). However, the influence emplacement (Chanceaux and Menand, 2014). Heterogeneities in the strength, of shallow-level, laterally extensive mafic sill complexes on volcanism and tec- Young’s modulus, and pore fluid pressure between rock units within layered tonic processes remains enigmatic. Here we summarize the key controls on strata can also instigate the rotation of the minimum compressive principal sill complex emplacement and discuss their potential implications for future stress axis to vertical orientations, promoting the deflection of ascending dikes volcanological studies. into strata-concordant sills (Anderson, 1951; Gretener, 1969; Kavanagh et al., 2006; Valentine and Krogh, 2006; Gressier et al., 2010; Gudmundsson, 2011; Sill Complex Formation Barnett and Gudmundsson, 2014; Kavanagh and Pavier, 2014). Magma emplacement facilitated by brittle failure of rock-rock interfaces or Mechanics of Sill Emplacement stiff rocks is commonly accommodated by (1) compaction of the overburden and/or underlying sedimentary strata (e.g., Morgan et al., 2008); (2) floor sub- Sill emplacement within sedimentary basins has been attributed to (see sidence (e.g., Cruden and McCaffrey, 2001); and/or (3) overburden uplift when Menand, 2011, and references therein) (1) the deflection of dikes into sills at a the magma pressure within a sill exceeds the lithostatic load (e.g., Pollard and depth where magma density equals that of the host rock (i.e., the level of neu- Johnson, 1973; Koch et al., 1981; Jackson and Pollard, 1990; Bunger and Cruden,

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Relative layer stiffness Inner-arc compression A B Outer-arc extension Layer 1 < Layer 2LLayer 1 = Layer 2 ayer 1 > Layer 2 Neutral surface Figure 16. (A) A plot depicting how the re- 2 lation between Dundurs elastic mismatch parameter, a quantitative measure of the difference in Young’s modulus between two layers, and the ratio of the energy

release rate (i.e., Gd /Gp) affects the geometry of a propagating dike upon reaching C a contact between two layers (modified

p from Gudmundsson, 2011; Barnett and G Dike penetration Gudmundsson, 2014). The parameters Gd

/ 1 d (i) and Gp correspond to the energy release G rate required for a vertically propagating fracture in Layer 2 to either penetrate Layer 1 (i) or deflect along the Layer 1 Single deflection (ii) D W E and 2 interface (ii and iii), respectively Inferred (Gudmundsson, 2011). (B) Extensional and compressional strains generated during (iii) Dike deflection Double deflection forced folding above intrusions with dome-shaped upper contacts (modified 0 from Cosgrove and Hillier, 1999). (C) For- -1 0 1 Ramp (inclined sheet) Dundurs elastic mismatch parameter mation of flat-topped forced folds and intrusions through concentrated strain Flat (sill) Layer 1 along the intrusion periphery, which may result in reverse faulting. (D) Example of a transgressive sheet intrusion on Ardna Bedding Ramp (inclined sheet) murchan, northwest Scotland, which con- Layer 2 1 m sists of sill and inclined sheet segments and overall exhibits a ramp-flat morphology (see Magee et al., 2012a). A schematic diagram is inset that highlights the open- (i) Layer 1 (ii) Layer 1 (iii) Layer 1 Sill ing vectors (black double-headed arrows) of the intrusion (modified from Muirhead et al., 2014). Inclined sheet Layer 2 Layer 2 Layer 2

2011; Agirrezabala, 2015). Of these three space-making mechanisms, only evi- within the host rock during this type of forced folding may promote tensile dence of roof uplift (i.e., forced folds above intrusions) is observed in seismic failure and/or normal faulting at the crest or periphery of the fold (Fig. 16B; e.g., reflection data (e.g., Figs. 3B, 3D, 8A, 8B, and 12B; e.g., Hansen and Cartwright, Magee et al., 2013a). In contrast to the formation of dome-shaped folds, nu- 2006b; Jackson et al., 2013; Magee et al., 2013a; Sun et al., 2014); poor seismic merical modeling suggests that flat-topped, steep-sided intrusions and asso imaging immediately beneath intrusions compromises our ability to discern ciated forced folds (Fig. 16C) may form over time as a consequence of fluid floor subsidence. The geometry of these intrusion-induced forced folds, which mechanics (e.g., the weight of the magma) and/or a heterogeneous magma broadly mirrors the morphology of the associated upper sill or laccolith con- pressure distribution (Bunger and Cruden, 2011; Galland and Scheibert, 2013). tact (e.g., Figs. 3B, 3D, 8A, 8B, and 12B), can provide further information on The formation of flat-topped forced folds, which may also be promoted by in- brittle emplacement conditions and mechanisms (Pollard and Johnson, 1973; trusion beneath a sequence of rigid strata that resist folding and layer-parallel Koch et al., 1981; Jackson and Pollard, 1988, 1990; Galland, 2012; Galland and slip, is likely to concentrate shear stresses near the intrusion tips and possibly Scheibert, 2013; van Wyk de Vries et al., 2014; Agirrezabala, 2015). For example, result in the nucleation of reverse faults that may be subsequently intruded dome-shaped forced folds developed above sills or laccoliths that taper toward (Fig. 16C; Thomson and Schofield, 2008; Bunger and Cruden, 2011; van Wyk their lateral tips (e.g., Figs. 3D, 12B, and 16B) require a homogeneous pres- de Vries et al., 2014). Because sill complexes comprise multiple intrusions that sure distribution within the intrusion and layer-parallel slip to occur along the may overlap laterally, the growth of individual forced folds may be inhibited or fold limbs (Pollard and Johnson, 1973; Koch et al., 1981; Jackson and Pollard, modified due to the impingement of neighboring folds; this could result in fold 1990; Galland and Scheibert, 2013). Outer arc extensional strains generated coalescence and the formation of compound folds (Fig. 8C; Magee et al., 2014).

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While the emplacement and accommodation of sheet intrusions is tra- Muirhead et al., 2012). Stress fields favoring classical cone sheet formation ditionally regarded as being controlled by linear elastic fracture mechanics (e.g., Anderson, 1937) have been reproduced numerically during inflation of (e.g., Pollard, 1973; Rubin, 1995; Malthe-Sørenssen et al., 2004; Kavanagh circular or oblate-shaped magma chambers (e.g., Gudmundsson, 2002, 2006; et al., 2006; Menand et al., 2010; Bunger and Cruden, 2011), field observations Bistacchi et al., 2012). Similarly, within sill complexes, numerical modeling has suggest that sills may intrude into shale, coal, or poorly consolidated beds via demonstrated that toward the lateral tips of a sill, during intrusion and/or infla-

nonbrittle processes (e.g., Schofield et al., 2012a; Muirhead et al. 2016). For tion, s1 is radially inclined inward and can thereby promote the transgression example, magmatic heating can cause coal or certain salts (e.g., carnallite) to of an inclined sheet (e.g., Gudmundsson, 2002, 2006; Malthe-Sørenssen et al., behave as a viscous fluid during intrusion, inhibiting the propagation of brittle 2004; Gudmundsson and Løtveit, 2012; Barnett and Gudmundsson, 2014). It fractures and instead promoting the formation of magma finger instabilities may therefore be expected that inclined sheets within sill complexes com- along the fluidal magma–host rock contact (e.g., Figs. 2B and 6; McClintock monly occur within extension fractures. and White, 2002; Schofield et al., 2012a, 2014). Similar pseudoviscous However, field kinematic studies indicate that some cone sheets (e.g., the fluid-fluid interfaces may be generated by the intrusion of magma into wet, Tejeda Complex, Canary Islands, Schirnick et al., 1999; Ardnamurchan, Scot- poorly consolidated host rocks at emplacement depths typically 500 m (Ein- land, Magee et al., 2012a) and sill-fed inclined sheet swarms (e.g., the Ferrar sele et al., 1980; Miles and Cartwright, 2010; Airoldi et al., 2011; Schofield et al., ≲ sill complex, Antarctica, Muirhead et al., 2012, 2014) emplaced as mixed- 2012a). The fluidization of intact rocks by the expansion and volatization of mode fractures. Although the initial stages of these inclined sheets were pore fluids or vapor decreases the cohesion between grains, and can also probably characterized by mode-I fracture propagation (i.e., Gudmundsson, promote the development of viscous fluid-fluid contacts between the magma 2002), field observations reveal that these intrusions transgress via a series and the host rock (Schofield et al., 2010, 2012a; Magee et al., 2014). Fluidization of small interconnected sill and inclined sheet segments (i.e., they have a may be triggered by magmatic heating and/or depressurization during rapid, ramp-flat morphology; Fig. 16D; Airoldi et al., 2011; Magee et al., 2012a; Muir- intrusion-induced failure of the host rock (Schofield et al., 2010, 2012a). These head et al., 2012). The intrusions thus exhibit two different components of nonbrittle mechanisms of host-rock deformation can accommodate some, if opening, dilation normal to the inclined sheet plane and subvertical open- not all, of the related magma volume through porosity reduction and plastic ing of the sill segments (Airoldi et al., 2011; Magee et al., 2012a; Muirhead deformation (e.g., Fig. 6A). The occurrence of multiple brittle and/or nonbrittle et al., 2012). The combination of these modes of opening results in intrusion space-making processes during emplacement may explain why forced fold propagation and growth under mixed-mode I-II fracturing, and produces a amplitudes measured in seismic data are commonly less than the intrusion component of reverse shear along the intrusion plane (Fig. 16D; Muirhead thickness (Fig. 13). et al., 2012, 2014; Mathieu et al., 2015). Because entire sill complexes, which consist of interconnected sills and inclined sheets, are comparable to the Inclined Sheet Formation smaller scale ramp-flat sheets described here (cf. Figs. 7C, 11B, and 16D), it may be expected that mixed-mode fracturing is prevalent in sill complex emplacement. Seismic data reveal that strata-concordant sills may transition laterally into transgressive inclined sheets, potentially forming saucer-shaped sills if There are several other mechanisms that may aid or promote the develop- the inclined sheets encompass the intrusion (Figs. 8–12 and 14; Thomson and ment of inclined sheets: (1) the generation of tensile strains and fractures at Hutton, 2004). These inclined sheets transfer magma from deeper sills to shal- the tips of sills in response to overburden bending (Figs. 8C and 16B; Jackson lower stratigraphic levels and are thus integral to facilitating magma ascent in and Pollard, 1990; Malthe-Sørenssen et al., 2004; Goulty and Schofield, 2008; sedimentary basins (Thomson and Hutton, 2004; Cartwright and Hansen, 2006; Thomson and Schofield, 2008; Muirhead et al., 2012; Galland and Scheibert, Holford et al., 2012; Muirhead et al., 2012; Magee et al., 2013c, 2014). Within the 2013; Magee et al., 2013a, 2014); (2) the emplacement of magma along reverse volcanological literature, studies of inclined sheet emplacement have primarily faults that nucleated during the growth of flat-topped forced folds (Fig. 16C; focused on the formation of cone sheets, i.e., swarms of concentric, inwardly Thomson and Schofield, 2008; Bunger and Cruden, 2011); (3) intrusion along inclined sheet intrusions that dip toward and are inferred to emanate from a preexisting faults (e.g., Fig. 14A; Bedard et al., 2012; Magee et al., 2013c; McClay central magma chamber (e.g., Anderson, 1937; Gautneb et al., 1989; Schirnick et al., 2013); (4) the intrusion of magma along inclined unconformity surfaces et al., 1999; Klausen, 2004; Geshi, 2005; Bistacchi et al., 2012; Burchardt et al., (C. McLean, personal observations); or (5) the upward deflection, along a 2013). Field observations reveal that the majority of these cone sheets occur curved trajectory, of a propagating fracture beyond the lateral tip of a sill in

in extension fractures and thereby intruded when s1 was radially inclined in- response to the asymmetrical emplacement of magma above the initial level

ward, parallel to the inclined sheet plane, while s3 was orthogonal to the intru- of the extension fracture (Hansen, 2015). In summary, magma ascent along sion plane (e.g., Anderson, 1937; Gautneb et al., 1989; Gudmundsson, 2002, inclined sheets in sill complexes is likely facilitated by multiple processes, and 2006; Klausen, 2004; Geshi, 2005; Siler and Karson, 2009; Magee et al., 2012a; therefore resolving the key controls on transgression is challenging.

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Role of Dikes largely inhibited (Pollard and Johnson, 1973). Given that the host rock–magma interactions facilitating mafic sill transgression (e.g., stress rotation) occur at An important consideration when examining sill complex emplacement the lateral tips of intermediate to felsic sills (e.g., Pollard and Johnson, 1973; concerns the role of dikes in facilitating magma transport between individ- Gudmundsson, 2012), it is likely that magma rheology is an important control ual sills, inclined sheets, and the surface. It is, however, difficult to assess the on whether a sill complex or laccolith develops (Johnson and Pollard, 1973; role of dikes within sill complexes using seismic data because the technique is McLeod and Tait, 1999; Currier and Marsh, 2015); i.e., it appears easier for mafic biased toward imaging subhorizontal to moderately inclined sheet intrusions magma, which typically has a higher temperature and lower viscosity than (Smallwood and Maresh, 2002; Thomson, 2007). Field observations indicate intermediate to felsic magma, to form inclined sheets at sill tips. Because the that dikes are volumetrically minor components of sill complexes (Fig. 7; e.g., rheology of a magma is a function of its chemistry, dissolved and exsolved the Karoo-Ferrar LIP; Leat, 2008; Hastie et al., 2014; Muirhead et al., 2014). Re- volatile content, crystal content, ascent/emplacement rate, and temperature cent analogue experiments conducted by Kavanagh et al. (2015) may provide (Pistone et al., 2012, 2013), the incremental development of a sill complex or an alternative explanation for the apparent lack of dikes in sill complexes. laccolith through the injection of distinct magma pulses will therefore be influ- The analogue models showed that magma pressure within underlying feeder enced by the magma input rate and its solidification dynamics (Johnson and dikes decreased upon the inception and propagation of a sill, promoting dike Pollard, 1973; Glazner et al., 2004; Currier and Marsh, 2015). While numerous contraction (Kavanagh et al., 2015). Although there is currently little field evi- studies have examined how magma rheology may affect the growth of an dence to support feeder dike contraction during sill emplacement, it may be individual intrusion (e.g., Johnson and Pollard, 1973; Taisne and Tait, 2011; Cur- considered that the complete or partial closure of dikes could serve to reduce rier and Marsh, 2015), our understanding of the influence of magma rheology their apparent volumetric importance in sill complexes. The role of dikes in on the development of entire shallow-level plumbing systems is limited and sedimentary basins may therefore be underestimated. However, it is important represents an important aspect that requires further research. to note that the majority of sills within a sill complex likely have thicknesses below the limit of detectability in seismic data, implying that the volume of sills Lateral Magma Flow may be underestimated (Button and Cawthorn, 2015; Schofield et al., 2015). Field- and seismic-based observations suggest that plumbing systems Influence of Magma Rheology and the Growth of Sill Complexes within sedimentary basins are typically dominated by interconnected sill complexes that transgress as much as 12 km of strata and extend across lateral Field and borehole data indicate that laterally extensive sill complexes distances of as much as 4100 km (e.g., Figs. 6–12, 14, and 15; Table 1; e.g., commonly have mafic compositions (e.g., Chevallier and Woodford, 1999; Aspler et al., 2002; Cartwright and Hansen, 2006; Leat, 2008; Svensen et al., Smallwood and Maresh, 2002; Leat, 2008; Wingate et al., 2008; Schofield et al., 2012; Jackson et al., 2013; Magee et al., 2014; Muirhead et al., 2014; Neres et al., 2015). In contrast, field observations reveal that the incremental injection of 2014; Schofield et al., 2015). While the source of numerous sill complexes re- more viscous magmas with evolved, intermediate to felsic compositions and mains poorly constrained, commonly due a paucity of seismic and/or bore- emplaced at shallow levels will tend to form laccoliths instead of sills (e.g., hole data in relevant areas, several studies have proposed that sill complexes Johnson and Pollard, 1973; John and Blundy, 1993; Cruden and Launeau, 1994; may (1) originate from underlying melt zones (high-velocity bodies in seismic Stevenson et al., 2007; Michel et al., 2008; Bunger and Cruden, 2011). Experi- data) via dikes, fault-hosted intrusions, or transgressive sills (e.g., Figs. 10A, mental and numerical modeling, coupled with field observations, indicate that 10B, and 11C; e.g., Cartwright and Hansen, 2006; Galerne et al., 2011; Rohrman, the emplacement of mafic sills and felsic laccoliths are both initially character- 2013; Schofield et al., 2015); (2) be fed by laterally propagating dike swarms ized by the intrusion and lateral propagation of a thin sill (e.g., Johnson and that emanate from volcanic centers or mantle plume heads (e.g., Hyndman Pollard, 1973; Cruden and McCaffrey, 2001; Leuthold et al., 2012; Kavanagh and Alt, 1987; Ernst et al., 1995); (3) propagate directly away from volcanic et al., 2006, 2015; Currier and Marsh, 2015). While these thin, mafic and inter- centers (Magee et al., 2014; Neres et al., 2014); or (4) be sourced directly from mediate to felsic sills will both subsequently undergo a phase of inflation, their a plume head, with lateral movement promoted by emplacement into an area final growth stage varies (e.g., Johnson and Pollard, 1973; Kavanagh et al., of topographic uplift above the plume head and the generation of a hydraulic 2006, 2015; Currier and Marsh, 2015). For example, observations presented gradient sufficient enough to allow sills (<10 m thick) to flow laterally for thou- here indicate that mafic sills commonly transition into inclined sheets at their sands of kilometers without freezing (Fialko and Rubin, 1999; Aspler et al., lateral tips (e.g., Malthe-Sørenssen et al., 2004; Thomson and Hutton, 2004; 2002; Leat, 2008). Polteau et al., 2008b; Magee et al., 2013c; Hansen, 2015). In contrast, laccoliths The distribution of sill complexes is also influenced by the structure and form through sill thickening in response to continued magma injection and stratigraphy of the basin (Schofield et al., 2012a; Magee et al., 2013c). In partic- overburden bending (Johnson and Pollard, 1973; Cruden and McCaffrey, 2001; ular, the presence of fault arrays in sedimentary basins can limit the areal ex- Menand, 2011), suggesting that inclined sheet intrusion at the lateral sill tips is tent of sill complexes by (1) providing suitable magma ascent pathways to the

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Earth’s surface or shallower stratigraphic levels where cooler host rocks facili- mal and chemical evolution (see Annen et al., 2015, and references therein). tate solidification (e.g., Fig. 14A) or (2) deflecting sill propagation, particularly If the duration between distinct pulses is too long, allowing the preexisting if footwall basement rocks are juxtaposed against hanging-wall sedimentary intrusion to cool and potentially solidify, any new batch of magma is effec- strata (e.g., Fig. 9C). Sill complexes emplaced within relatively unstructured tively intruded into a relatively low-temperature environment (Annen, 2011). basins (e.g., the Karoo Basin), however, cover areas that are orders of magni- The accretion of new magma above or below a preexisting sill or stack of sills tude larger than those intruded into faulted basins; e.g., the Karoo sill complex within a relatively low-temperature environment will therefore simply serve covers ~3 × 106 km2 (Fig. 7A), whereas sill complexes across the Møre and to thicken the igneous body, potentially forming laccoliths, without signifi- Vøring Basins only cover ~8 × 104 km2 (Fig. 11A; cf. Planke et al., 2005; Leat, cant lateral propagation (e.g., John and Blundy, 1993; Menand, 2008; Annen, 2008; Svensen et al., 2012; Magee et al., 2014; Schofield et al., 2015). 2011; Broderick et al., 2015). This thickening and stacking of sills (i.e., little or no lateral growth) may be aided by the contraction of preexisting intrusion Duration of Sill Complex Emplacement components during solidification, generating space for new, distinct magma batches (John and Blundy, 1993). If, however, the duration between the injec- Sill complexes represent tortuous magma ascent pathways compared to tion of distinct magma pulses is short and/or there is sufficient latent heat of swarms of dikes. It may therefore be expected that magma migration, from crystallization, the maintenance of locally elevated temperatures may allow melt source to the Earth’s surface, will take longer if facilitated by sill, as op- later magma pulses to remain hotter for longer, facilitating intrusion further posed to dike, emplacement. The duration of sill complex development may up into the stratigraphic sequence (Fig. 17A; e.g., Glazner et al., 2004; Annen, thus prolong magma residence times, affecting geochemical and petrological 2011; Annen et al., 2015). The accretion of sills would result in an increase in processes. For sill complexes where absolute age data are available or rela- the apparent total thickness of intruded material (Fig. 17A), possibly producing tive dating techniques can be employed, studies suggest that magma injection discernable top and basal seismic reflections. However, the vast majority of occurs rapidly over a few million years (Skogseid et al., 1992; Archer et al., sills imaged in seismic data are expressed as tuned reflection packages and it 2005; Planke et al., 2005; Cartwright and Hansen, 2006; Hansen and Cartwright, is thus difficult to determine if mapped reflectors correspond to one or multi- 2006b; Hansen, 2006; Svensen et al., 2012; Schofield and Jolley, 2013). For ex- ple intrusions (e.g., Figs. 6–12 and 14). ample, radiometric dating suggests that sills within the Karoo magmatic prov- An alternative explanation concerns the development and prolonged ince were emplaced over 0.47 m.y. at a rate of 0.78 km3/yr (Svensen et al., 2012). longevity of discrete magma flow pathways within individual intrusions (Fig. However, in Magee et al. (2014), multiple onlap relationships in forced folded 17B). Numerous studies have shown that sills commonly form through the co- successions above a sill complex in the Rockall Basin were shown, suggesting alescence of magma segments or fingers and, on a larger scale, magma lobes that fold growth, and thereby sill emplacement, occurred incrementally over (e.g., Figs. 4 and 5; e.g., Pollard et al., 1975; Thomson and Hutton, 2004; Scho- ~15 m.y. through the periodic injection of small magma batches (Figs. 8A, 8B). field et al., 2010, 2012a, 2012b). The channelization of magma into one or sev- Evidence for the prolonged development of a sill complex in the Rockall Basin eral of these zones can increase the segment thickness, allowing magma flow therefore implies that seismic-stratigraphic or radiometric dates obtained from to be sustained while neighboring portions of the intrusion crystallize (Fig. 17B; only one or several intrusions in a sill complex, which most previous studies Holness and Humphreys, 2003). If these channelized segments remain par- relied upon, cannot reliably be applied to the entire magmatic network (Magee tially molten, through the continued injection of small magma batches and/or et al., 2014; Evenchick et al., 2015). insulation by adjacent lobes, subsequent magma pulses may preferentially Numerous studies have documented that individual sills and laccoliths, as reactivate the channels (Fig. 17B; Currier and Marsh, 2015). Flow of magma well as entire intrusion networks, may develop through the emplacement of through thin channels, which then bud off to form larger sills, is supported by distinct magma pulses, the duration between which is highly variable (e.g., mapped magma flow patterns that suggest that sills are intruded from a point- Coleman et al., 2004; Glazner et al., 2004; Michel et al., 2008; Miller et al., like source (e.g., Fig. 5; Thomson and Hutton, 2004; Magee et al., 2013c, 2014; 2011; Magee et al., 2014; Annen et al., 2015; Broderick et al., 2015; Currier and Schofield et al., 2015). Marsh, 2015). If some sill complexes are emplaced incrementally but over time frames in which magma solidification should theoretically occur, how then Implications for Volcano Monitoring does magma rheology vary and how do magma migration pathways remain operational within these systems? Consider a scenario whereby new distinct Understanding volcano histories and tracking real-time magma movement magma pulses may intrude along the contact, which likely represents a strong within the subsurface is crucial to the appraisal and mitigation of volcanic mechanical discontinuity, between fully or partially solidified intrusions and hazards (Sparks, 2003; Sparks et al., 2012; Cashman and Sparks, 2013). Tradi- the host rock (i.e., underaccretion or overaccretion; Fig. 17A; see Annen et al., tionally, volcano data (e.g., geochemistry, petrology, geophysics) are generally 2015, and references therein). This stacking and amalgamation of sills through interpreted within the framework of a vertically stacked magma plumbing sys- repeated magma injection along sill–host rock contacts can influence its ther- tem where the eruption site overlies the melt source (e.g., Fig. 1A; Cashman

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A Key Paleosurface

t1 Magma pulse 1

t2 Magma pulse 2 me Ti t3 Magma pulse 3 me higher sustained Ti temperatur e t4 Magma pulse 4

Figure 17. (A) Schematic diagram demon- strating how the emplacement of one t B 1 Magma pulse 1 magma pulse may facilitate the intrusion

t2 Magma pulse 2 of later pulses further up into the sedimen- Magma fingers tary sequence if accretion allows sills to remain hotter for longer (Annen et al., 2015). (B) Sketch depicting how discrete magma segments and/or lobes may preferentially Inclined limb composed remain open as magma channels (magma of magma fingers pulse 2), feeding stratigraphically higher intrusions, when adjacent portions of the sill have crystallized (magma pulse 1).

Inner sill

Magma pulse 1 lobes

and Sparks, 2013; Tibaldi, 2015). However, this review documents that sill com- sites. The possibility that some volcanoes and volcano fields, regardless of the plexes, consisting of interconnected sills and inclined sheets, can cover broad tectonic setting, are the product of a laterally extensive and interconnected areas and feed volumetrically important eruptions (e.g., Fig. 1B; e.g., Planke magma plumbing system therefore needs to be incorporated into volcano et al., 2005; Leat, 2008; Svensen et al., 2012; Magee et al., 2014; Muirhead logical studies. et al., 2014; Schofield et al., 2015). Furthermore, observations from offshore To emphasize how the consideration of a laterally arranged magma plumb- Norway and onshore Australia suggest that sill complexes may develop in ing system is integral to volcanic hazard assessment, we briefly discuss how crystalline continental crust (Figs. 11C and 15; Cartwright and Hansen, 2006). sill complex development may affect ground deformation patterns. The analy These observations of lateral magma flow within sill complexes, which extend sis of ground deformation patterns, recorded using InSAR and tilt meters, is as much as 4100 km from the melt source and intrude a variety of host rocks, commonly applied to tracking the migration and volume of magma in volcanic imply that melt processes cannot be mapped from the distribution of eruption systems (e.g., Sparks, 2003; Wright et al., 2006; Biggs et al., 2009, 2011; Sparks

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et al., 2012). Seismic-based studies highlight that eruption sites may be offset thinning into the Danakil Depression, Bastow and Keir (2011) proposed that the from shallow-level magma reservoirs and that magma migration through a sill final stages of continent-ocean transition in Ethiopia are being characterized complex follows a convoluted route (e.g., Fig. 12; e.g., Jackson, 2012; Magee not by a shift to magma intrusion-dominated extension, but by a late stage et al., 2013b). Uplift and subsidence patterns expressed at the Earth’s surface of plate stretching and sedimentary basin formation. For dikes to reach the and associated with magma movement within a sill complex may therefore surface in the Danakil Depression, or in other sedimentary basins developed occur away from volcanoes. For example, geodetic and seismicity studies indi- along the East African Rift system, they are likely to interact with subhorizontal cate that the magma reservoir for recent eruptions at Asama Volcano (Japan) strata and may be expected to deflect into sills (e.g., Schofield et al., 2014). was located 8 km to the west of the summit (Aoki et al., 2013). Perhaps the To date, InSAR studies have recorded the emplacement of individual sill-like most important implication to studies of ground deformation, however, is that intrusions within rift basins, such as the Danakil Depression, along the East magma intrusion is not necessarily accommodated by roof uplift (e.g., Fig. African Rift system (Biggs et al., 2009, 2011, 2013; Pagli et al., 2012). It is there- 13; Schofield et al., 2012a; Magee et al., 2013a). Instead, multiple space-mak- fore worth investigating whether these observed sill emplacement events cor- ing processes can accommodate magma emplacement, some of which may respond to the growth of a sill complex, particularly when considering that sill produce no discernible ground deformation (e.g., porosity reduction). We sug- complexes can host significant volumes of magma and facilitate long-distance gest that ground deformation inversions are therefore likely to underestimate magma transport (Leat, 2008; Svensen et al., 2012). magma volumes (Galland, 2012). In extreme circumstances where magma emplacement occurs via nonbrittle fluidization of the host rock and no overburden deformation occurs (e.g., Fig. 6A; Schofield et al., 2012a, 2014), the intruded CONCLUSIONS magma volume would be hidden from ground deformation, and potentially seismicity, monitoring techniques. The structure of magma plumbing systems influences tectonic and vol canic processes, and is traditionally considered to be dominated by the ver- Implications for Continental Break-Up tical intrusion of dikes. Delimiting the true structure of plumbing systems is, however, challenging due to the accessibility of active systems and limitations Continental rifting and break-up are commonly associated with the forma- in exposure at the Earth’s surface. Seismic reflection data have arguably revo tion of sedimentary basins and the generation of large, mafic melt volumes. lutionized our understanding of plumbing systems and, in conjunction with Ethiopia is an ideal natural laboratory to study magmatic plumbing system field observations, have revealed that interconnected sills and inclined sheets structure during continental break-up because ongoing earthquakes and vol- (i.e., a sill complex) can facilitate the magma transport over vertical distances canic eruptions associated with the East African Rift system mean that the of 12 km and for as much as ~4100 km laterally. Volcanoes fed by sill com- interaction between tectonic and magmatic processes can be observed and plexes may thus be laterally offset from the melt source. These sill complexes ≲ constrained (e.g., Ebinger and Casey, 2001; Keir et al., 2006; Wright et al., 2006; preferentially form in sedimentary basins, although they may also occur within Corti, 2009; Ebinger et al., 2010). Based on a variety of disciplines, a consensus crystalline continental crust. In particular, mechanical contrasts across rock- has emerged that Quaternary magmatism in Ethiopia is almost entirely domi rock interfaces (e.g., elastic mismatch) and the presence of compliant rock nated by diking, which accommodates extension (e.g., Ebinger and Casey, units, which may be intruded in a nonbrittle fashion (e.g., fluidization of shale), 2001; Keir et al., 2006; Wright et al., 2006, 2012; Corti, 2009; Ebinger et al., 2010). promote sill emplacement and control the structure of sill complexes. Magma The importance of dike intrusion in Ethiopia is supported by (1) recording of 14 is accommodated via a combination of roof uplift, which may produce dis- individual dike intrusions during the 2005–2010 Dabbahu-Manda-Harraro rift- cernible ground deformation, and other space-making processes such as po- ing event (Ebinger et al., 2010; Wright et al., 2012); (2) measurements of seismic rosity reduction. The occurrence of multiple accommodation mechanisms im- anisotropy from the Main Ethiopia Rift that suggest the presence of aligned plies that ground deformation may not correlate to the entire magma volume melt bodies, interpreted as swarms of dikes (Keir et al., 2005; Kendall et al., emplaced and, in extreme circumstances, no uplift may be associated with 2005; Bastow et al., 2010); and (3) volcanic cones in the Main Ethiopia Rift that shallow-level sill intrusion. These space-making processes compromise our are aligned along underlying dikes (Korme et al., 1997; Keir et al., 2015; Muir- ability to estimate magma volumes and locations using ground deformation head et al., 2015) and exhibit geochemical variations consistent with variable analyses. The presence of a sill complex emplaced into continental crystalline and heterogeneous mantle sources (e.g., Rooney et al., 2011, 2012). Farther crust in the Yilgarn craton highlights that extensive lateral magma transport north of the Main Ethiopian Rift, however, where the Red Sea Rift descends to- in sills could feasibly occur in any host rock and tectonic setting. Furthermore, ward and below sea level in the Danakil Depression (i.e., a sedimentary basin), the extent to which active volcanic systems and rifted margins are underlain the style of volcanism changes; i.e., Quaternary–Holocene volumes of fissure by networks of sills remains unknown. We conclude that it is crucial to con- basalt flows are significantly greater than to the south (e.g., Barberi and Varet, sider that plumbing systems may be characterized by sill complexes when 1970). On the strength of this evidence, and the observation of abrupt crustal constraining magmatic processes, melt volumes, and melt sources.

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ACKNOWLEDGMENTS Barrett, P., 1981, History of the Ross Sea region during the deposition of the Beacon Supergroup 400–180 million years ago: Royal Society of New Zealand Journal, v. 11, p. 447–458, doi:10 We benefitted from discussions with many Earth scientists in different disciplines over the years; .1080/03036758.1981.10423334. we particularly thank Ken Thomson, Donny Hutton, Brian O’Driscoll, Mike Petronis, Ken McDer- Bastow, I.D., and Keir, D., 2011, The protracted development of the continent-ocean transition in mott, Derek Keir, Ben van Wyk de Vries, and Davie Brown for their insights. We thank Schlum- Afar: Nature Geoscience, v. 4, p. 248–250, doi:10 .1038 /ngeo1095 . berger for software and data provision, and Department of Communications, Energy, and Natu- Bastow, I., Pilidou, S., Kendall, J.M., and Stuart, G., 2010, Melt-induced seismic anisotropy and ral Resources (Petroleum Affairs Division) in Ireland, Geoscience Australia, and PGS (Petroleum magma assisted rifting in Ethiopia: Evidence from surface waves: Geochemistry, Geo Geo-Services) for provision of seismic data. This work was completed as part of Magee’s Junior physics, Geosystems, v. 11, Q0AB05, doi:10.1029 /2010GC003036 . Research Fellowship funded by Imperial College London. Muirhead acknowledges support from Bedard, J.H., et al., 2012, Fault-mediated melt ascent in a Neoproterozoic continental flood basalt Fulbright New Zealand and the Ministry of Science and Innovation. We thank Shan de Silva for his province, the Franklin sills, Victoria Island, Canada: Geological Society of America Bulletin, editorial handling of the manuscript and Tyrone Rooney, Agust Gudmundsson, and Mattia Pistone v. 124, p. 723–736, doi:10.1130/B30450.1. for the time and effort they put in to their constructive reviews. Biggs, J., Anthony, E., and Ebinger, C., 2009, Multiple inflation and deflation events at Kenyan volcanoes, East African Rift: Geology, v. 37, p. 979–982, doi:10 .1130 /G30133A .1 . 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